libwebp: update to v0.5.1

third_party/libwebp/enc/backward_references.h

Index: third_party/libwebp/enc/backward_references.h
diff --git a/third_party/libwebp/enc/backward_references.h b/third_party/libwebp/enc/backward_references.h
index daa084d84681b54d022ae0d0dd59176683178a8a..0cadb11e11ef421d5099aa9037f98be8f936b28f 100644
--- a/third_party/libwebp/enc/backward_references.h
+++ b/third_party/libwebp/enc/backward_references.h
@@ -115,11 +115,12 @@ static WEBP_INLINE uint32_t PixOrCopyDistance(const PixOrCopy* const p) {
 
 typedef struct VP8LHashChain VP8LHashChain;
 struct VP8LHashChain {
-  // Stores the most recently added position with the given hash value.
-  int32_t hash_to_first_index_[HASH_SIZE];
-  // chain_[pos] stores the previous position with the same hash value
-  // for every pixel in the image.
-  int32_t* chain_;
+  // The 20 most significant bits contain the offset at which the best match
+  // is found. These 20 bits are the limit defined by GetWindowSizeForHashChain
+  // (through WINDOW_SIZE = 1<<20).
+  // The lower 12 bits contain the length of the match. The 12 bit limit is
+  // defined in MaxFindCopyLength with MAX_LENGTH=4096.
+  uint32_t* offset_length_;
   // This is the maximum size of the hash_chain that can be constructed.
   // Typically this is the pixel count (width x height) for a given image.
   int size_;
@@ -127,6 +128,9 @@ struct VP8LHashChain {
 
 // Must be called first, to set size.
 int VP8LHashChainInit(VP8LHashChain* const p, int size);
+// Pre-compute the best matches for argb.
+int VP8LHashChainFill(VP8LHashChain* const p, int quality,
+                      const uint32_t* const argb, int xsize, int ysize);
 void VP8LHashChainClear(VP8LHashChain* const p);  // release memory
 
 // -----------------------------------------------------------------------------
@@ -192,8 +196,8 @@ static WEBP_INLINE void VP8LRefsCursorNext(VP8LRefsCursor* const c) {
 // refs[0] or refs[1].
 VP8LBackwardRefs* VP8LGetBackwardReferences(
     int width, int height, const uint32_t* const argb, int quality,
-    int low_effort, int* const cache_bits, VP8LHashChain* const hash_chain,
-    VP8LBackwardRefs refs[2]);
+    int low_effort, int* const cache_bits,
+    const VP8LHashChain* const hash_chain, VP8LBackwardRefs refs[2]);
 
 #ifdef __cplusplus
 }