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

src/enc/backward_references.h

@@ -185,21 +185,16 @@ static WEBP_INLINE void VP8LRefsCursorNext(VP8LRefsCursor* const c) {
 // Main entry points
 
 // Evaluates best possible backward references for specified quality.
-// Further optimize for 2D locality if use_2d_locality flag is set.
+// The input cache_bits to 'VP8LGetBackwardReferences' sets the maximum cache
+// bits to use (passing 0 implies disabling the local color cache).
+// The optimal cache bits is evaluated and set for the *cache_bits parameter.
 // The return value is the pointer to the best of the two backward refs viz,
 // refs[0] or refs[1].
 VP8LBackwardRefs* VP8LGetBackwardReferences(
     int width, int height, const uint32_t* const argb, int quality,
-    int cache_bits, int use_2d_locality, VP8LHashChain* const hash_chain,
+    int* cache_bits, VP8LHashChain* const hash_chain,
     VP8LBackwardRefs refs[2]);
 
-// Produce an estimate for a good color cache size for the image.
-int VP8LCalculateEstimateForCacheSize(const uint32_t* const argb,
-                                      int xsize, int ysize, int quality,
-                                      VP8LHashChain* const hash_chain,
-                                      VP8LBackwardRefs* const ref,
-                                      int* const best_cache_bits);
-
 #ifdef __cplusplus
 }
 #endif