add WebPMuxSetCanvasSize() to the mux API

previously, the final canvas size was adjusted tightly from the
animation frames. Now, it can be specified separately (to be larger, in particular).

calling WebPMuxSetCanvasSize(mux, 0, 0) triggers the 'adjust tightly' behaviour.
This can be useful after calling WebPMuxCreate() if further image addition
is expected.

-> Fixed gif2webp accordingly.

also: made WebPMuxAssemble() more robust by systematically zero-ing WebPData.

Change-Id: Ib4f7eac372cf9dbf6e25cd686a77960e386a0b7f

src/mux/muxi.h

@@ -65,7 +65,9 @@ struct WebPMux {
   WebPChunk*      anim_;
   WebPChunk*      vp8x_;
 
-  WebPChunk*  unknown_;
+  WebPChunk*      unknown_;
+  int             canvas_width_;
+  int             canvas_height_;
 };
 
 // CHUNK_INDEX enum: used for indexing within 'kChunks' (defined below) only.