Speed up lossless encoder.

Speedup lossless encoder by 20-25% by optimizing:
- GetBestColorTransformForTile: Use techniques like binary search and
  local minima search to reduce the search space.
- VP8LFastSLog2Slow & VP8LFastLog2Slow: Adding the correction factor for
  log(1 + x) and increase the threshold for calling the approximate
  version of log_2 (compared to costly call to log()).

Change-Id: Ia2444c914521ac298492aafa458e617028fc2f9d