Palette: Don't use top-right pixel for context of color indices.
This reduces the complexity in a number of ways: - We need just 3 neighbors instead of 4. - Possible contexts reduce from 16 to 5. - On hardware side, getting the contexts for a whole block will be more parallelizable. At the same time, compression performance improves very slightly: - Screen-content set (videos) (Google): BDRate improved by 0.32 - screenshots set (images) (AWCY): PSNR improved by 0.62: https://arewecompressedyet.com/?job=palette_withTR2%402017-01-27T21%3A30%3A28.890Z&job=palette_noTR2%402017-01-27T21%3A41%3A34.312Z Change-Id: Ie84ca32f05d55ad481a51c2d3abc579468597189
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