The present invention relates to a process for manufacturing plasma display panels with distributed getter material; the invention relates also to the displays obtained according to the process of the invention. Plasma display panels are known under the abbreviation PDP, which will be used in the following. A PDP is composed of two planar glass parts, a front one and a rear one, sealed at their perimeter by a low-melting point glass paste. In this way between the two glass parts a closed space is formed, filled with a rare gas mixture and comprising functional components, as specified in the following; generally the rare gas mixture is composed of neon and xenon, with the latter one is present in a quantity between about 4 and 15%. The working principle of a PDP is based on the conversion into visible light, by the so-called phosphors, of ultraviolet radiations when an electric discharge is generated in the rare gas mixture. In order to form an image, a plurality of light sources of small dimensions is necessary, and thus a plurality of electrodes which generate localized discharges; every light source formed in this way is defined in the field "pixel". Figures 1 and 2 show in cross-section, respectively, a part of a known PDP and of its front glass panel only (the relative dimensions are not in scale); in particular, the two views are taken along two mutually orthogonal sections. On the front glass panel, FP, is present a series of pairs of parallel electrodes, E1 and E2, defined as supporting electrodes and as scanning electrodes respectively, being protected by a dielectric layer, FD, which in turn is covered with a layer, M, of magnesium oxide (MgO); this latter has the double function of protecting the dielectric layer from the ionic bombardment due to the plasma triggered by the discharge, and of supplying secondary electrons for maintaining the discharge. On the rear glass panel, RP, a series of so-called address electrodes, AE, is present (having a direction orthogonal to the electrodes E1 and E2), covered by a dielectric layer, RD; a series of barriers R (known in the field as "ribs") that are mutually parallel and parallel to the electrodes AE, is constructed onto this latter layer. Since the internal pressure of the display is lower than atmospheric pressure, the upper portion of the ribs is in contact with layer M, thus dividing the inner space of the display into parallel channels, indicated as C in the drawing, having a width between 0.1 and 0.3 mm. Each one of these channels is covered internally with phosphors; particularly, in the channels there are present in an alternating way phosphors, able to convert ultraviolet light respectively into red (phosphors PR), green (PG) and blue (PB) visible light. By applying a potential difference to a given electrode pair E1 and E2 and to an electrode AE, an electric discharge is generated in the zone of a pixel, that causes the light emission indicated by the arrows in figure 1. The area of the front glass panel, corresponding to the zone of the channels, is the part on which the image is formed. Recently, interfering effects between the electric discharges at contiguous pixels in one channel have been noticed (a phenomenon known as "cross- talking"), which cause a deterioration of the image quality, especially in the case of high-definition displays (i.e. having pixels of small dimensions). In order to reduce the phenomenon, more complex configurations of the ribs have been proposed, such as shown in the figures 3 to 5. hi the case of figure 3 the channels are divided transversally by barriers' of a height that is lower than that of the ribs; in the case of figure 4 the ribs define pixels of essentially hexagonal geometry, separated by necks with a reduced cross section; finally, figure 5 shows a structure in which there are transversal barriers of the same height as the ribs, so that the inner space of the display results divided in ordered rows of completely closed cells (each one corresponding to a pixel). The manufacturing processes of PDPs are essentially of two types, i.e. the so-called "pumping tubulation" processes, currently used, or the "chamber processes", under investigation, hi a process of the pumping tabulation type, in one of the two glass panels forming the display (usually the rear panel) an opening is formed, connected to a glass tubulation; after the perimetral sealing of the perimeter of the two glass panels, first the evacuation of the inner space is carried out by pumping through the tabulation and subsequently said inner space is filled with the desired rare gas mixture; finally the tubulation is closed by compression under heat, thus sealing the inner space of the display. Instead, in a chamber process the two finished glass panels are introduced into a chamber filled with an atmosphere having composition and pressure corresponding to that of the rare gas mixture required for operating the PDP, and sealed to each other in this chamber, to enclose the appropriate atmosphere. Consequently, in the case of the pumping tabulation processes the filling of the display with the gas mixture follows the sealing of the two glass panels, while in the case of the chamber processes the two steps are simultaneous. It must be noted that while generally the choice of either process is free, in the particular case of the displays with an internal structure with closed cells, as that shown in figure 5, it is necessary to resort to the chamber process, because after the sealing of the two glass panels it would not be possible any more to evacuate the cells or to fill them with the rare gas mixture via the tubulation. For the proper operation of these devices it is necessary that the chemical composition of the gaseous mixture in which the plasma is formed remains constant: in fact, the presence in the gaseous mixture of traces of atmospheric gases as nitrogen, ■ oxygen, water or carbon oxides, has the effect to vary the operating electrical parameters of the PDP, as discussed in the articles "Effect of reactive gas dopants on the MgO surface in AC plasma display panels", by W. E. Ahearn et al, published in "IBM J. RES. DEVELOP." Vol. 22, No. 6 (1978), p. 622; "Color plasma displays: status of cell structure designs" by H. Doyeux, published in SID 00 Digest, p. 212; and "Relationships between impurity gas and luminance/discharge characteristics of AC PDP" by J.-E. Heo et al., published in "Journal of Information Display", Vol. 2, No. 4 (2001), p. 29. In particular, among PDP manufacturers, water is the impurity regarded as the most dangerous one. These impurities can remain in the panel following the manufacturing process, or they can accumulate at the inside with time, as a consequence of outgassing of the component materials themselves. The first contribution is particularly important in the case of the pumping tabulation processes, in which the limiting factor for the evacuation speed of the inner space is the low gas conductance in the channels, which causes that the removal of the impurities cannot be completed within the evacuation times (some hours) compatible with the industrial manufacturing processes of PDPs; the problem is even worse in the case of PDPs with internal structures like those shown in figures 3 and 4 (while as already stated, displays with a structure of type 5 cannot be produced in this way). The contribution from the outgassing during the service life is instead the same in PDPs produced by the two methods. In order to solve these problems it has been proposed to introduce in various ways into the PDPs getter materials, i.e. materials capable to react with the impurities and to chemically fix them, thus removing them definitely from the inner space of these displays. Patent US 6,472,819, patent application US-A1-2003-0071579 and Korean patent application KR-Al -2001 -104469 disclose PDPs in which getter material deposits are present in the peripheral zone, within the sealing zone between the front and rear glass panels and the image-forming zone. The getter deposits according to these documents are efficient both in increasing the removal speed of the impurities during the manufacturing process of the display, and in removing the impurities generated by outgassing during the service life thereof. In spite of the advantages offered, the getter systems according to these documents do not yet yield totally satisfying results; in fact, particularly during the service life of the display, the impurities need some time to reach the getter materials, during which inhomogeneity of gas composition across the PDP may arise and consequently differences in luminosity or in image quality at different parts of the display. To overcome the problem, some patent documents describe various configurations in which the getter material is distributed in the image forming area. Korean patent No. 366095 and Korean patent application KR-A1-2001- 049126 describe PDPs in which linear getter material deposits, parallel to the electrodes (of the type E1 and E2 in figure 1), are present on the front glass panel, so that the getter deposits also form the so-called "black matrix" of the display (a dark element surrounding the pixels that increases the contrast of the display). However, in the structures described in these documents the getter deposits cover part of the surface dedicated to the light emission and thus an extremely precise control of dimensions and location of these deposits is required, with quite complex manufacturing processes; moreover, at least in the case of Korean patent 366095, the surface of the getter deposits forms an undercut with respect to the surface of the magnesium oxide layer, whereby every getter deposit provides for a possible communication passage for the gases between contiguous channels, with a possible increase of the cross-talking. Patent US 6,483,238 Bl and Japanese patent application JP-A1-2002- 075170 disclose PDPs in which the ribs are made from a porous material, containing the getter material, while the Korean patent application KR- Al -2001- 091313 discloses a PDP in which the ribs are made from getter material. These structures however show some constructive problems, in so far as the ribs are generally constructed by successive depositions of a suspension of particles of the desired material with the screen-printing technique, drying after every layer deposition, and final consolidation of the rib by thermal treatment; the use of a mixture of various materials, among which a getter, gives some problems, since the getter could be contaminated during the thermal treatments of drying and consolidation by the vapors of the solvent used for the deposition, thus resulting inactive for the service life of the display; vice versa the presence of getter particles could compromise the mutual adhesion of the particles of ceramic material of which the ribs are normally formed, thus reducing their mechanical resistance. Finally, patent US 6,603,260 Bl discloses a PDP in which a getter material is deposited on the upper surface of the ribs, in contact with the front glass panel. However, also this solution presents notable constructive difficulties, in fact, in order to deposit the getter selectively only on the upper surface of the ribs, extremely precise masking operations are necessary, in order to avoid that the material spreads along the lateral walls and occupies the zone designated for the phosphors (or to cover them, in case these are already present). Object of the present invention is to overcome the shortcomings of the prior art, in particular to provide a simple manufacturing process for producing a plasma display panel containing a distributed getter. This and other objects are obtained according to the present invention, with a manufacturing process for plasma display panels comprising the following steps: - manufacturing a front glass panel of a plasma display panel provided with pairs of supporting electrodes and scanning electrodes, a layer of dielectric material for the protection of said electrodes and a layer of magnesium oxide which covers the layer of dielectric material; - manufacturing a rear glass panel of a plasma display panel provided with barriers designed to define channels or cells in the finished display, address electrodes and phosphors; - sealing along the outer perimeter of said front and rear glass panels, thus defining a closed space or a plurality of closed spaces inside the display; - filling said spaces with a rare gas mixture necessary, for the operation of the display; characterized in that before said sealing step, on the free surface of said magnesium oxide layer, getter material deposits are formed at positions essentially corresponding to the contact areas between said front glass panel and the barriers on the rear glass panel. The invention will be described in the following with reference to the drawings wherein: Fig. 1 shows a cross sectional view taken perpendicularly to the channels of a prior art plasma display panel; Fig. 2 shows a partial view of a cross-section orthogonal to that of Fig. 1, of only the front glass panel of a prior art plasma display panel; Figs. 3 to 5 show some particular embodiments of the ribs that define the channels or cells of displays known in the art; - Fig. 6 shows in a view similar to that of figure 2, the main operational steps characterizing the process of the invention in a first embodiment thereof; Fig. 7, similar to Fig; 6, shows the main operational steps characterizing the process of the invention in an alternative embodiment; Fig. 8 shows in a cross sectional view similar to Fig. 1, a plasma display panel of the invention in its most general embodiment; and - Fig. 9 shows in a view similar to that of Fig. 8, a plasma display panel according to an alternative embodiment. The figures 1 to 5 have been described in the introduction. The process of the invention is different from the known processes only in that the manufacturing of the front glass panel comprises the steps of forming a number of getter deposits on the surface, that in the finished display is facing the inner space, at locations essentially corresponding to the contact areas with the upper portion of the ribs. The getter deposits may be formed either on the plane surface of the MgO layer (M in figure 1) or into recesses formed in this layer. The invention is applicable indifferently to either pumping tabulation or in chamber manufacturing processes of PDPs. Figure 6 shows the various steps of the operation characterizing the invention (in this drawing, the front glass panel is shown upside down with respect to figures 1-5). During step a, above the surface of the magnesium oxide layer onto which the getter deposits have to be formed, a mask 60 is aligned, provided with apertures 61, 61', ..., that geometrically correspond to the zones where the front glass panel will contact the upper portion of the ribs in the finished display; for clarity of the drawing mask 60 is shown spaced apart from the surface of layer M, but it could be in contact therewith. In step b, particles (generally referred to as element 62) of getter material are brought in various ways, according to the adopted deposition technique, onto the upper surface of the mask 60, and reach the free surface of the layer M only in the zones of the apertures 61, 61', ... . Finally, in step c, the deposits 63, 63', ... of getter material particles have been formed; these deposits may or may not require thermal treatments for consolidation, depending on the deposition process. Figure 7, similar to figure 6, shows the various steps of the additional operation characterizing the invention in an alternative embodiment. In this case the free surface of the MgO layer has recesses 71, 71', ... corresponding to the apertures 61, 61', ... of the mask 60; these recesses may be obtained either during the formation of layer M, or by selective removal of material from the layer M, for example by ion bombardment, using in this operation (not shown in the drawing) the same mask 60; the recesses 71, 71', ... shown in the drawing extend only within layer M, but could also pass through it and reach the underlying layer DF. Step a' corresponds to step a of the first embodiment, with the only difference that in this case a higher precision in the alignment of the mask 60 with respect to the surface of the layer M is required. The following steps b' and c' are similar to the steps b and c of the first embodiment, resulting in the formation of the getter material deposits 72, 72', .... Preferably, step b' has a longer duration than step b, in order to allow the complete filling of the recesses 71, 71', ... and the formation of deposits 72, 72', ... of such a height to protrude from the free surface of the layer M (thus obtaining a similar result to the deposits 63, 63', ,..); this has the effect of favoring the contact between the gases to be sorbed and the lateral surfaces of the deposits 72, 72', ... in the finished display. The material and the construction of mask 60, and the distance between the mask and layer M during the deposition of the getter material particles 62, depend on the adopted deposition technique, which itself can depend on the nature of the material to be deposited. As said in the introduction, the main impurity to be sorbed is water, whereby it is possible to use a moisture sorbing material as getter. The preferred materials to this effect are the oxides of alkaline-earth metals, which react with water according to the reaction: MO + H2O ■» M(OH)2 wherein M can be calcium, strontium or barium; it is also possible to use mixtures of these oxides, possibly with addition of magnesium oxide. For the manufacturing of the deposits (63, 63', ...; 72, 72', ...) of these oxides it is possible to use various techniques, among which, for example, screen- printing, sputtering, Chemical Vapor Deposition (CVD), or Electron Beam Evaporation. The technique of screen-printing is well-known in the field of reproduction of patterns on textiles, ceramics or other materials, and is described in case of the preparation of getter deposits, for example, in the patent US 5,882,727, to which it is referred for details. In this case the mask 60 consists of a net with openings selectively blocked by a polymeric material, leaving clear the openings corresponding to the apertures 61, 61', .„; then a suspension of the material particles to be deposited is prepared in a suitable suspension medium; the mask is preferably laid onto the layer M of the front glass panel, the suspension is distributed onto the net and forced to pass to the underlying support, in correspondence with said apertures. In the specific case of the present invention the suspension medium obviously cannot be water-based (as common in other applications of the technique) because of the nature of the materials to deposit,' whereby organic solvents can be used such as liquid hydrocarbons at room temperature. It is particularly easy to produce mixed deposits with this technique, starting from a mixture of different oxide particles. The techniques of sputtering, CVD and electron beam evaporation are widely used in the microelectronics industry and are well known to the technicians of the field, not requiring further illustrations, hi this case the mask 60 can be a discrete element, for example a metallic foil with holes corresponding to the apertures 61, 61 ', ...; or, as widely known in the field, it is possible to use a polymeric deposit formed onto layer M, in which the apertures are formed by sensitization with UV light and subsequent chemical attack to the sensitized zones; after the formation of deposits 63, 63', ... or 72, 72', ..., all polymeric material is removed using a chemical attack, different from the first one. In the case of sputtering the deposition of one or more oxides can be obtained either starting directly from targets of oxides, or starting from metal targets by operating in the so-called "reactive sputtering" conditions, i.e. with a small percentage of oxygen in the reaction atmosphere. In the case of CVD the substrate is held at a temperature sufficiently high to decompose the organic component carrying the interested metal and in an oxidizing atmosphere, so that the decomposition of the organic precursor and the formation of the oxide occur at the same time; in this case it is particularly easy to form a mixed oxide, because it is sufficient to transport a mixture of vapors composed of precursors of the different metals onto the substrate (the layer M). Finally, in the case of electron beam evaporation, it is sufficient to subject to electron bombardment a material (or a mixture of materials) corresponding to the material intended for the deposit; this material (or mixture) can, for example, be contained in a crucible with the upper surface open, placed in the same chamber as the support on which the deposits are to be formed. For the sorption of impurities different from water it is possible to form deposits of non evaporable getter metals or alloys. These materials (known as NEG) are widely employed for the sorption of reactive gases in all applications where it is required to maintain vacuum or the purity of inert gasses. Examples of these materials are the metals titanium and zirconium or their alloys with one or more elements selected from the transition metals and aluminum, hi particular the alloys Zr-Al can be mentioned, described in the patent US 3,203,901, and in particular the alloy with weight percent composition Zr 84% - Al 16%, manufactured and sold by the applicant under the trade name St 101; the alloys Zr-V-Fe described in the patent US 4,312,669, and in particular the alloy with weight percent composition Zr 70% -V 24.6% - Fe 5.4%, manufactured and sold by the applicant under the trade name St 707; and the ternary alloys Zr-Co-A (where A indicates an element selected from yttrium, lanthanum, rare earths or their mixtures) described in the patent US 5,961,750, and in particular the alloy with weight percent composition Zr 80.8% -Co 14.2% - A 5%, manufactured and sold by the applicant under the trade name St 787. Deposits of these materials are preferably produced by sputtering or electron beam evaporation. Figure 8 shows a cross sectional view, taken perpendicularly to the direction of the channels, of a plasma display panel according the invention, 80, in its most general embodiment, in which by 81, 81', ..., deposits of getter material are indicated, independently of the nature of the latter The NEG materials operate better at relative high temperatures, for example, over 300 °C, and are therefore active mainly during the manufacturing process of the PDP, during the general heating steps to which the components of the display are subjected to favor outgassing or for the sealing of the two (front and rear) glass panels. Vice versa, moisture sorbing materials work better at room temperature, and in the case of calcium oxide, at the temperatures occurring during the manufacturing process of the PDP water could even be released. Consequently, NEGs are more useful for the removal of the impurities during the manufacturing of the PDP, while moisture sorbers are more useful during the service life thereof. Considering that the two types of material are complementary, it is also possible according to the process of the invention to foresee the formation of alternating deposits of moisture sorbing material and NEG. Figure 9 shows this alternative possibility in a view similar to that of figure 8: in display 90, the deposits of a moisture sorbing material, 91, 91', ..., are alternated to deposits of a NEG material, 92, 92', ... IQ this way every channel (or cell) of the PDP is exposed to a surface of both materials, so that the NEG contributes to keep clean the internal atmosphere of that channel (or cell) during the manufacturing of the PDP, also sorbing water possibly released from the moisture sorber during this step, whereas the moisture sorber performs the function of removing the water from each channel (or cell) during the service life of the PDP. To obtain this configuration it can be sufficient to manufacture the deposits of the two different materials, e.g. by sputtering, in two subsequent deposition phases, taking care of moving the mask 60 between the two phases for a step as large as the distance between two contiguous ribs. In any case it can be preferable to operate in such a way to produce not too compact getter deposits, because the presence of porosities in these deposits increases the surface of material in contact with the gases and as a consequence the sorption properties, particularly the speed. One way of producing by sputtering NEG deposits particularly effective for the sorption of gases is described in the European patent application EP 1518599 A2 the name of the applicant. Preferably the getter deposits, either of oxides of alkaline-earth metals or of NEG materials, are produced with the same technique with which the MgO layer of the front glass panel is produced, to limit the number of transfers to different process chambers which generally are laborious and affect the cost of the whole process. In a further variant, it is possible to add titanium dioxide, TiO2, to the getter materials; it is in fact known that this material, when irradiated with UV radiation, is able to catalytically convert hydrocarbons into simpler species, and in the presence of oxygenated gases to water and CO2. Due to the low efficiency of hydrocarbon sorption by the getter materials, the addition of TiO2 in a plasma display panel (which innerly produces UV radiation during its operation) allows to convert these species into other ones, more efficiently sorbed. hi the case of deposits of moisture sorbing material formed for example by screen-printing, it is possible to add TiO2 particles to the initial suspension; in the other cases, a TiO2 deposit is preferably added on the getter material deposit (so that in the finished display it is in contact with the ribs) or underneath the same (so that it is between the getter material and magnesium oxide). With the process of the invention the introduction of getter material in a PDP results easy, because it allows to loosen the requirements regarding dimensions and localisation of the deposits of such a material, hi particular, the difficulties encountered with patent US 6,603,260 Bl in depositing the getter onto the ribs with precise alignment and dimensioning, are avoided. These advantages are particularly appreciable when PDPs have to be produced with complex shapes of the ribs as in the case illustrated in figure 4.