Mux API change:

'Set' and 'Get' methods for images take/return a bitstream as input,
instead of separate 'image' and 'alpha' arguments.

Also,
- Make WebPDataCopy() a public API
- Use WebPData for storing data in WebPChunk.
- Fix a potential memleak.

Change-Id: I4bf5ee6b39971384cb124b5b43921c27e9aabf3e

examples/vwebp.c

@@ -162,7 +162,7 @@ static int Decode(const int frame_number, uint32_t* const duration) {
 
   ClearPreviousPic();
   if (kParams.has_animation) {
-    if (WebPMuxGetFrame(kParams.mux, frame_number, &image_data, NULL,
+    if (WebPMuxGetFrame(kParams.mux, frame_number, &image_data,
                         &x_off, &y_off, duration) != WEBP_MUX_OK) {
       goto end;
     }