gif2webp: Add a mixed compression mode

When '-mixed' option is given, each frame would be heuristically chosen
to be
encoded using lossy or lossless compression.

The heuristic is based on the number of colors in the image:
- If num_colors <= 31, pick lossless compression
- If num_colors >= 194, pick lossy compression
- Otherwise, try both and pick the one that compresses better.

Change-Id: I908c73493ddc38e8db35b7b1959300569e6d3a97

examples/gif2webp_util.h

@@ -44,9 +44,11 @@ typedef struct WebPFrameCache WebPFrameCache;
 
 // Given the minimum distance between key frames 'kmin' and maximum distance
 // between key frames 'kmax', returns an appropriately allocated cache object.
+// If 'allow_mixed' is true, the subsequent calls to WebPFrameCacheAddFrame()
+// will heuristically pick lossy or lossless compression for each frame.
 // Use WebPFrameCacheDelete() to deallocate the 'cache'.
 WebPFrameCache* WebPFrameCacheNew(int width, int height,
-                                  size_t kmin, size_t kmax);
+                                  size_t kmin, size_t kmax, int allow_mixed);
 
 // Release all the frame data from 'cache' and free 'cache'.
 void WebPFrameCacheDelete(WebPFrameCache* const cache);