libwebp: update to v0.5.1

third_party/libwebp/enc/alpha.c

Index: third_party/libwebp/enc/alpha.c
diff --git a/third_party/libwebp/enc/alpha.c b/third_party/libwebp/enc/alpha.c
index 3c970b00b12648ef4cb0ee57bbb6e9ae3b3790b8..03e3ad07f5f86743c32370946724fcd6bf52afbb 100644
--- a/third_party/libwebp/enc/alpha.c
+++ b/third_party/libwebp/enc/alpha.c
@@ -79,7 +79,11 @@ static int EncodeLossless(const uint8_t* const data, int width, int height,
   config.quality = 8.f * effort_level;
   assert(config.quality >= 0 && config.quality <= 100.f);
 
-  ok = (VP8LEncodeStream(&config, &picture, bw) == VP8_ENC_OK);
+  // TODO(urvang): Temporary fix to avoid generating images that trigger
+  // a decoder bug related to alpha with color cache.
+  // See: https://code.google.com/p/webp/issues/detail?id=239
+  // Need to re-enable this later.
+  ok = (VP8LEncodeStream(&config, &picture, bw, 0 /*use_cache*/) == VP8_ENC_OK);
   WebPPictureFree(&picture);
   ok = ok && !bw->error_;
   if (!ok) {
@@ -118,7 +122,6 @@ static int EncodeAlphaInternal(const uint8_t* const data, int width, int height,
   assert(method >= ALPHA_NO_COMPRESSION);
   assert(method <= ALPHA_LOSSLESS_COMPRESSION);
   assert(sizeof(header) == ALPHA_HEADER_LEN);
-  // TODO(skal): have a common function and #define's to validate alpha params.
 
   filter_func = WebPFilters[filter];
   if (filter_func != NULL) {