Updated VP8LGetBackwardReferences and color cache.

- The optimal cache bits is evaluated inside the method 'VP8LGetBackwardReferences'.
- The input cache_bits to 'VP8LGetBackwardReferences' sets the maximum cache
  bits to use (passing 0 implies disabling the local color cache).
- The local color cache is disabled for lowerf (<= 25) quality levels (as before).
- Enabled local color cache for palette images as well. This saves additional
  0.017% bytes with a slight (2-3%) improvement in the compression speed.
- Removed 'use_2d_locality' parameter from method VP8LGetBackwardReferences, as
  this option is not an option now (after we freeze the lossless bit-stream).

Change-Id: I33430401e465474fa1be899f330387cd2b466280
Merge "Updated VP8LGetBackwardReferences and color cache."