Backlighting Unit, Display Screen, and Method of Manufacture The present invention relates to an active privacy backlight, also termed a mode- switchable backlighting unit, and a display screen. Background to the Invention In the automotive field, there is a need for automotive displays to have a switchable privacy mode for some use experiences, for example, for preventing the driver from being distracted by the content of the display. Such displays preferably have a switchable beam angle, which allows the screen to be viewable or switched to a mode where they are not viewable from certain positions. By way of an example, in one mode the screen may be viewable by the driver and the passenger, but in another mode the screen may be viewable by only the passenger. Displays in the art typically use a two-layer, edge-lit light-guide design. For example, International patent publication number WO 2020185264 describes a mode-switchable backlight comprising a first directional backlight comprising an array of narrow-angle emitters configured to provide narrow-angle emitted light during both a first mode and a second mode; and a second directional backlight comprising an array of bidirectional emitters configured to provide bidirectional emitted light exclusively during the second mode, the bidirectional emitted light having a bifurcated angular extent that is complementary to an angular range of the narrow-angle emitted light. A combination of the narrow-angle emitted light and the bidirectional emitted light during the second mode is configured to provide broad-angle emitted light having an angular range that is a summation of the angular range of the narrow-angle emitted light and the bifurcated angular extent of the bidirectional emitted light. US patent publication number US 2018259799 describes a display device having a privacy mode and a public mode, which uses a light blocking arrangement (comprising elements formed from a photochromic material) to block laterally directed light in the privacy mode, but not in the public mode. The displays described in the above documents have significant cost, thickness, and/or power efficiency issues. Summary of the Invention There is generally a need for an apparatus and method which addresses one or more of the problems identified above. It is an object of an aspect of the present invention to provide a mode-switchable backlighting unit that obviates or at least mitigates one or more of the aforesaid disadvantages of the arrangements known in the art. Further aims and objects of the invention will become apparent from reading the following description. According to a first aspect of the invention, there is provided a mode-switchable backlighting unit comprising: a. a first array of one or more light sources; b. a second array of one or more light sources, independent from the first array of one or more light sources; and c. a transparent light guide for guiding light emitted from the first array and second array of light sources, the transparent light guide comprising an array of extraction features which form a light output surface of the mode-switchable backlighting unit, the extraction features comprising an asymmetric shape, wherein the mode-switchable backlighting unit has: a first mode of operation, corresponding to the first array of one or more light sources illuminating the array of extraction features to provide an output beam from the light output surface having a first angle of divergence; and a second mode of operation, corresponding to the second array of one or more light sources illuminating the array of extraction features to provide an output beam from the light output surface having a second angle of divergence, the second angle of divergence being larger than the first angle of divergence. The mode-switchable backlighting unit of the present invention advantageously allows for at least two different and switchable beam angle distributions (i.e. the angle of divergence) to be delivered by a single light-guide backlighting unit. The angle of divergence is preferably quantified by the full width at half maximum (FWHM) of the output beam. Compared to arrangements in the art, the backlighting unit of the present invention has a lower cost (because of fewer components and reduced complexity) and easier system integration. Furthermore, the backlighting unit of the present invention is more energy efficient than arrangements in the art. This is because the backlighting unit can have fewer layers; compared to, for example, International patent publication number WO2020185264, the number of light guide layers can be reduced from two to one. This yields a higher optical efficiency and thus a lower system power consumption. Additionally, it follows that the backlighting unit of the present invention is advantageously thinner than arrangements in the art. By "independent", in the context of the first array and second array of one or more light sources, it is meant that the second array of one or more light sources can be switched on and off without affecting the first array of one or more light sources, and vice versa. In other words, the first array of one or more light sources can be on while the second array of one or more light sources is off, and vice versa. By “transparent”, in the context of the light guide, it is meant that the rate of visible light transmission is more than 80%, and preferably more than 90%, according to the test standard ASTM D-1003 (Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics). The extraction features may comprise a first surface profile and a second surface profile. Light from the first array of one or more light sources may preferably interact substantially with the first surface profile. Light from the second array of one or more light sources may preferably interact substantially with the second surface profile. By “substantially”, in this context, is it meant that at least 85%, or at least 90%, or at least 95%, of light emitted from the one or more lights sources interacts with the surface profile. The first angle of divergence may be less than about 90°, or less than about 80°, or less than about 70°, or less than about 60°, or about 40°. The second angle of divergence may be greater than about 90°, or greater than about 100°, or about 110°. The relative normal intensity in the first mode of operation may be approximately double the relative normal intensity in the second mode of operation. The first array of one or more light sources and the second array of one or more light sources may preferably be coplanar. Additionally, the first array and the second array of one or more light sources may preferably be coplanar with the transparent light guide. Preferably, the backlighting unit comprises a single transparent light guide (i.e. no more than one transparent light guide). This advantageously provides a backlighting unit that is thinner than those disclosed in the art. The transparent light guide may comprise a layer of transparent sheet, and a layer of the array of extraction features may be mounted on the layer of transparent sheet. In some embodiments, the extraction features comprise refractive extraction features. In some embodiments, the extraction features comprise reflective extraction features. The extraction features may comprise a transparent prism structure, wherein at least one face of the prism is curved. In these embodiments, the curved face of the prism is the second surface profile, and the flat/straight face of the prism is the first surface profile. The mode-switchable backlighting unit may be absent of any symmetric extraction features. The one or more light sources can be any of those known to those skilled in the art, particularly those which are suitable for use in backlighting. Such light sources include one or more light emitting diodes (LEDs), cold cathode fluorescent lamps, laser diodes, organic light emitting diode sources, and other electroluminescent devices. The light may be non-directional. Preferably, the light sources comprise LEDs. The LEDs can be any of the designs known to those skilled in the art, including edge-emitting, side emitting, top emitting, or bare die LEDs. The mode-switchable backlighting unit may further comprise a substrate for supporting the first array of one or more light sources and the second array of one or more light sources. The transparent light guide may be disposed on the substrate. The transparent light guide may have a width of from about 0.1 mm to about 10 mm. Preferably, the transparent light guide is planar. The extraction features may have a height of from about 1 µm to about 100 µm. The mode-switchable backlighting unit may have a width of from about 0.1 mm to about 10 mm. The light sources may be encapsulated within the transparent light guide and/or in air pockets within the transparent light guide Alternatively, or additionally, the light sources may be adjacent the transparent light guide. Such embodiments may be termed "edge-lit". Alternatively, or additionally, the light sources may be distributed throughout the transparent light guide. For instance, each light source (from left-to-right in the transparent light guide) may alternate between the first array and the second array (i.e. in a A-B-A-B- A-B arrangement). Such embodiments may be termed “full array lit”. This advantageously provides different angles of light emission from the light sources. The mode-switchable backlighting unit may further comprise a specular reflector. The purpose of the reflector is to reflect the output beam back through the backlighting unit. The reflector may be positioned such that the array of extraction features is positioned between the reflector and the transparent light guide. Alternatively, the reflector may be positioned adjacent a surface of the transparent light guide opposite the light output surface. The mode-switchable backlighting unit may further comprise a diffuser, which advantageously improves the output beam quality. The diffuser may be positioned between the light output surface and the viewer of the mode-switchable backlighting unit, and in some instances adjacent the light output surface. The diffuser may be a weak gaussian diffuser. Embodiments comprising a diffuser and/or a reflector may be considered less preferred, because of the concomitant increase in thickness of the backlighting unit. However, the advantages provided by the diffuser and/or a reflector may be of particular benefit in certain applications. For instance, it is preferable for the mode-switchable backlighting unit to comprise a specular reflector when refractive light extraction features are used. According to a second aspect of the invention, there is provided a display screen comprising an image displaying element and a mode-switchable backlighting unit as hereinbefore described, the mode-switchable backlighting unit disposed under the image displaying element for backlighting the image displaying element. The display screen of the present invention, with a switchable beam angle, advantageously allows the screen to be viewable or switched to a mode where it is not viewable from certain positions. Thus, the display screen of the present invention is suitable for switching between a public mode and a privacy mode, while being cheaper and more energy efficient than display screens in the art. Thus, the display may be an automotive display. The image displaying element may comprise a thin-film-transistor (TFT) display. The display device may be a liquid crystal display device and may therefore comprise a liquid crystal cell which may also be referred to as a liquid crystal panel. Embodiments of the second aspect of the invention may include one or more features of the first aspect of the invention or its embodiments, or vice versa. According to a third aspect of the invention, there is provided a use of a mode-switchable backlighting unit as hereinbefore described, or a display screen as hereinbefore described, in an automotive vehicle. The inventors have found the present invention to be particularly beneficial to the automotive field. Embodiments of the third aspect of the invention may include one or more features of the first or second aspects of the invention or their embodiments, or vice versa. According to a fourth aspect of the invention, there is provided a method of manufacturing a mode-switchable backlighting unit as hereinbefore described, comprising the steps of: · providing a first array of one or more light sources and a second array of one or more light sources, independent from the first array of one or more light sources; · providing a transparent light guide suitable for guiding light emitted from the first array and second array of light sources; and · locating an array of extraction features on the transparent light guide, which form a light output surface of the mode-switchable backlighting unit, the extraction features comprising an asymmetric shape, wherein the mode-switchable backlighting unit has: a first mode of operation, corresponding to the first array of one or more light sources illuminating the array of extraction features to provide an output beam from the light output surface having a first angle of divergence; and a second mode of operation, corresponding to the second array of one or more light sources illuminating the array of extraction features to provide an output beam from the light output surface having a second angle of divergence, the second angle of divergence being larger than the first angle of divergence. The mode-switchable backlighting unit may be manufacturing by replicating a master. The master may be made using one or more of lasering, 3D photolithography and diamond tooling. The master may be replicated using one or more of embossing (including UV embossing) and micro-stamping. Alternatively, the mode-switchable backlighting unit may be manufactured using any suitable method known in the art for micro-optical films. Suitable materials for the transparent light guide, the array of extraction features, and (if present) the substrate, are conventional and would be known to the skilled person. The method may also comprise forming a display device from the mode-switchable backlighting unit. Embodiments of the fourth aspect of the invention may include one or more features of the first to third aspects of the invention or their embodiments, or vice versa. According to a fifth aspect of the invention, there is provided a mode-switchable backlighting unit comprising: a. a first array of one or more light sources; b. a second array of one or more light sources, independent from the first array of one or more light sources; and c. a transparent light guide for guiding light emitted from the first array and second array of light sources, the transparent light guide comprising an array of extraction features which form a light output surface of the mode-switchable backlighting unit, the extraction features comprising an asymmetric shape, wherein a light beam emitted from the output surface has a first angle of divergence when the array of extraction features is illuminated by the first array of one or more light sources, and the light beam emitted from the output surface has a second angle of divergence, larger than the first angle of divergence, when the array of extraction features is illuminated by the second array of one or more light sources. The mode-switchable backlighting unit advantageously can toggle between two modes: one having the first angle of divergence, and another having the second angle of divergence. In some embodiments of the fifth aspect, the mode-switchable backlighting unit has a first mode of operation, corresponding to the first array of one or more light sources illuminating the array of extraction features to provide an output beam from the light output surface having the first angle of divergence; and a second mode of operation, corresponding to the second array of one or more light sources illuminating the array of extraction features to provide an output beam from the light output surface having the second angle of divergence, the second angle of divergence being larger than the first angle of divergence. Embodiments of the fifth aspect of the invention may include one or more features of the first to fourth aspects of the invention or their embodiments, or vice versa. Brief Description of the Drawings There will now be described, by way of example only, various embodiments of the invention with reference to the drawings, of which: Figure 1 is a side view of a mode-switchable backlighting unit in accordance with an embodiment of the invention; Figure 2 is a side view of the mode-switchable backlighting unit in Figure 1 in the first mode of operation; Figure 3 is a side view of the mode-switchable backlighting unit in Figure 1 in the second mode of operation; Figure 4 is a side view of a mode-switchable backlighting unit in accordance with an embodiment of the invention; Figure 5 is a side view of a mode-switchable backlighting unit in accordance with an embodiment of the invention; Figure 6 is a side view of a mode-switchable backlighting unit in accordance with an embodiment of the invention; Figure 7 is a side view of a display screen in accordance with an embodiment of the invention; Figure 8 is a side view of example surface profiles which provide an output beam having a first angle of divergence; and Figure 9 is a side view of example surface profiles which provide an output beam having a second angle of divergence. Detailed Description of the Preferred Embodiments An explanation of the present invention will now be described with reference to Figures 1 to 9. Referring to Figure 1, a side view of a mode-switchable backlighting unit 1a in accordance with an embodiment of the present invention is presented. The mode-switchable backlighting unit 1a can be seen to comprise a first array of light sources 2, in the form of light-emitting diodes (LEDs), and a second array of light sources 3, also in the form of LEDs. The first array of light sources 2 and the second array of light sources 3 are coplanar. Both the first array of light sources 2 and second array of light sources 3 are edge coupled to a transparent planar light guide 4. Light is guided inside the transparent light guide 4 until it meets a change in the surface profile. A surface of the transparent light guide 4 forms a light output surface 5 of the mode-switchable backlighting unit 1a. The thickness of the transparent light guide 4 is in the range of 0.1 mm to 10 mm. Located across an outer surface of the transparent light guide 4 is an array of extraction features 6 having an asymmetric shape. In this embodiment, each extraction feature 6 has a transparent asymmetric prism structure, such that at least one face of the prism is straight 7 and at least one face of the prism is curved 8. Each extraction feature 6 on the transparent light guide 4 is identical. However, it will be appreciated that the extraction features 6 need not be identical, so long as each extraction feature 6 has an asymmetric shape. In this embodiment, the extraction features 6 are refractive extraction features and thus are located across the light output surface 5. However, it will be appreciated that the extraction features 6 may be reflective extraction features and thus can be located on a surface of the transparent light guide 4 opposite the light output surface 5. Figure 2 presents a side view of the mode-switchable backlighting unit 1a in Figure 1 in the first mode of operation. In this mode, the first array of light sources 2 is switched on and the second array of light sources 3 is switched off. When switched on, the first array of light sources 2 illuminates the array of extraction features 6, the light interacting with the straight face 7 of the extraction features 6 to provide an output beam 9 from the light output surface 5 having a first angle of divergence (α). Figure 3 presents a side view of the mode-switchable backlighting unit 1a in Figure 1 in the second mode of operation. In this mode, the second array of light sources 3 is switched on and the first array of light sources 2 is switched off. When switched on, the second array of light sources 3 illuminates the array of extraction features 6, the light interacting with the curved face 8 of the extraction features 6 to provide an output beam 9 from the light output surface 5 having a second angle of divergence (β). As can be seen comparing Figure 3 to Figure 2, the second angle of divergence (β) is larger than the first angle of divergence (α). Thus, the angle of divergence can be toggled between a relatively wide angle (as in Figure 3) and a relatively narrow angle (as in Figure 2) through selective illumination of the array of light sources 2,3 (that is, which array is switched on and which array is switched off). In these embodiments, the first angle of divergence (α) has a full width at half maximum (FWHM) of ± 20° (i.e. α equals 40°), and the second angle of divergence (β) has a FWHM of ± 55° (i.e. α equals 110°). However, it will be appreciated that the first and second angles of divergence need not be exactly these values, so long as the second angle of divergence (β) is larger than the first angle of divergence (α). Figure 4 presents a side view of a mode-switchable backlighting unit 1b in accordance with an embodiment of the present invention. This embodiment includes all the features of the embodiment presented in Figure 1 and operates in a similar way. However, instead of the first 2 and second 3 array of light sources being positioned adjacent the transparent light guide 4 (i.e. edge-lit), the light sources 2,3 are encapsulated within the transparent light guide 4. It will be appreciated that the light sources 2,3 could alternatively be located in air pockets within the transparent light guide 4, or a combination of both, or a combination of all three (i.e. encapsulated, air pockets and edge-lit). Figure 5 presents a side view of a mode-switchable backlighting unit 1c in accordance with an embodiment of the present invention. This embodiment includes all the features of the embodiment presented in Figure 4 and operates in a similar way. However, the mode- switchable backlighting unit 1c further comprises a substrate 10 for supporting the first array of one or more light sources 2 and the second array of one or more light sources 3. The transparent light guide 4 is disposed on the support 10, and the light sources 2,3 are positioned on the support 10, such that the light sources 2,3 are encapsulated by a combination of the support 10 and the transparent light guide 4. Figure 6 presents a side view of a mode-switchable backlighting unit 1d in accordance with an embodiment of the present invention. This embodiment includes all the features of the embodiment presented in Figure 4 and operates in a similar way. However, the first array 2 and second array 3 of one or more light sources are distributed throughout the transparent light guide 4. Such an arrangement is termed "full array". In this embodiment, each light source 2,3 (from left-to-right in the transparent light guide) alternates between the first array and the second array (i.e. in a A-B-A-B-A-B arrangement). Distributing the light sources 2,3 throughout the transparent light guide 4 provides different angles of light emission. Figure 7 presents a side view of a display screen 11 in accordance with an embodiment of the invention. The display screen comprises the mode-switchable backlighting unit 1 as shown in Figure 1 and a liquid crystal panel 12. It will be appreciated that any of the mode-switchable backlighting units described herein can be used with a liquid crystal panel 12. The liquid crystal panel 12 is positioned adjacent to the light output surface 5 of the mode-switchable backlighting unit 1. The arrow 13 indicates the direction in which the display screen 11 is viewed. Thus, the mode-switchable backlighting unit 1 illuminates the liquid crystal panel 12. By switching between the two modes of operation, the range in which the display screen 11 can be viewed is changed. That is, the display screen 11 can be switched to a mode in which the display screen 11 is not viewable from certain positions. In addition, the mode-switchable backlighting unit 1 comprises a specular reflector 14 adjacent a surface of the transparent light guide 4 opposite the light output surface 5. The purpose of the specular reflector 14 is to ensure that as much light as possible (preferably all the light) is directed through the liquid crystal panel 12. Figures 8A and 8B present side views of example surface profiles which provide an output beam 9 having a first angle of divergence (α). Both have a flat surface which yields light with a relatively narrow angle of divergence. Figures 9A and 9B present side views of example surface profiles which provide an output beam 9 having a second angle of divergence (β). Figure 9A shows a smooth curved surface which yields non-scattered light with a relatively wide angle of divergence. Figure 9B shows a jagged curved surface which yields scattered light with a relatively wide angle of divergence. Typically, each extraction feature 6 will comprise a surface profile as shown in Figure 8A or 8B, and a surface profile as shown in Figure 9A or 9B, such that the extraction feature 6 has an overall asymmetric shape. As discussed previously, the mode-switchable backlighting unit 1 has several advantages over the prior art. The backlighting unit 1 of the present invention has a lower cost (because of fewer components and reduced complexity), easier system integration, is thinner, and is more energy efficient than arrangements in the art. This is because the backlighting unit 1 has fewer layers. This yields a higher optical efficiency and thus a lower system power consumption. A mode-switchable backlighting unit 1 is disclosed, which comprises a first 2 and second 3 array of one or more lights sources and a transparent light guide 4, with the transparent light guide 4 comprising an array of extraction features 6 which form a light output surface 5, the extraction features 6 comprising an asymmetric shape. The mode-switchable backlighting unit 1 has at least two switchable modes of operation which provide an output beam 9 having a first (α) and second (β) angle of divergence, the second angle of divergence (β) being larger than the first angle of divergence (α). This allows for at least two different and switchable beam angle distributions to be delivered by a single light- guide backlighting unit 1. The mode-switchable backlighting unit 1 has several advantages, including reduced thickness and improved energy efficiency. Throughout the specification, unless the context demands otherwise, the terms “comprise” or “include”, or variations such as “comprises” or “comprising”, “includes” or “including” will be understood to imply the inclusion of a stated integer or group of integers, but not the exclusion of any other integer or group of integers. Furthermore, unless the context clearly demands otherwise, the term “or” will be interpreted as being inclusive not exclusive. The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. The described embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilise the invention in various embodiments and with various modifications as are suited to the particular use contemplated. Therefore, further modifications or improvements may be incorporated without departing from the scope of the invention as defined by the appended claims.