Add Alpha Decode support from WebPDecode.

Extend WebP decode functionality to extract Alpha data (support ALPH
chunk) and decode the Alpha plane using Alpha (utils/alpha) core-lib.

Change-Id: I6f0ee1b189c13caba3c1dd9b681383bfd00aa212

src/dec/vp8i.h

@@ -273,7 +273,7 @@ struct VP8Decoder {
 
   // extensions
   const uint8_t* alpha_data_;   // compressed alpha data (if present)
-  size_t alpha_data_size_;
+  uint32_t alpha_data_size_;
   uint8_t* alpha_plane_;        // output
 
   int layer_colorspace_;