MIPS: dspr2: added optimization for function FTransform

Change-Id: Ib5850edbc2a586ec9781f494b2337f024e22af78
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."
WebPEncode: Support encoding same pic twice (even if modified)

This wasn't working for this specific scenario:
- Encode an RGBA 'pic' (with trivial alpha) using lossy encoding.
(so that pic->a == NULL after import happens).
- Modify the 'pic->argb' so that it has non-trivial alpha.
- Encode the same 'pic' again.
This used to fail to encode alpha data as pic->a == NULL.

Change-Id: Ieaaa7bd09825c42f54fbd99e6781d98f0b19cc0c
Merge "WebPEncode: Support encoding same pic twice (even if modified)"