swscale: add two spatially stable dithering methods
Both of these dithering methods are from http://pippin.gimp.org/a_dither/ for GIF they can be considered better than bayer (provides more gray-levels), and spatial stability - often more than twice as good compression and less visual flicker than error diffusion methods (the methods also avoids error-shadow artifacts of diffusion dithers). These methods are similar to blue/green noise type dither masks; but are simple enough to generate their mask on the fly. They are still research work in progress; though more expensive to generate masks (which can be used in a LUT) like 'void and cluster' and similar methods will yield superior results
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