Palette Optimization: O(1) context lookup.
Now that we have small number of contexts (5), use hash multipliers (instead of base 11), so that color context hash is within a small range. This allows us to use a lookup table to get color context instead of a for loop. Output bitstreams are bit-exact, so no change in compression. Change-Id: I8cd8c893048c2fc6b22ccbd56f652d11486e2ee9