gif2webp: detect and flatten uniformly similar blocks

helps during lossless compression.

10% average saving, but that's mostly on what was previously
'difficult' cases, where the gain is ~30-50% actually.
Non-difficult cases are mostly unchanged.
Tested over ~7k random web gifs.

Change-Id: I09db4560e4ab09105d1cad28e6dbf83842eda8e9

examples/gif2webp.c

@@ -170,6 +170,7 @@ static int OptimizeAndEncodeFrame(
       // For lossy compression, it's better to replace transparent pixels of
       // 'curr' with actual RGB values, whenever possible.
       WebPUtilReduceTransparency(prev_canvas, &rect, curr);
+      WebPUtilFlattenSimilarBlocks(prev_canvas, &rect, curr);
     }
     if (!WebPFrameCacheShouldTryKeyFrame(cache)) {
       // Add this as a frame rectangle.