Add incremental support for extended format files

This applies to images with optional chunks (e.g. images with ALPH
chunk,
ICCP chunk etc). Before this, the incremental decoding used to work like
non-incremental decoding for such files, that is, no rows were decoded
until
all data was available.

The change is in 2 parts:
- During optional chunk parsing, don't wait for the full VP8/VP8L chunk.
- Remap 'alpha_data' pointer whenever a new buffer is allocated/used in
WebPIAppend() and WebPIUpdate().

Change-Id: I6cfd6ca1f334b9c6610fcbf662cd85fa494f2a91
(cherry picked from commit ead4d47859)

src/dec/vp8i.h

@@ -276,6 +276,7 @@ struct VP8Decoder {
   // extensions
   const uint8_t* alpha_data_;   // compressed alpha data (if present)
   size_t alpha_data_size_;
+  int is_alpha_decoded_;  // true if alpha_data_ is decoded in alpha_plane_
   uint8_t* alpha_plane_;        // output. Persistent, contains the whole data.
 
   int layer_colorspace_;