@TargetApi(12)
    private static void setAlpha(Bitmap bitmap) {
        if (Build.VERSION.SDK_INT >= 12) {
            bitmap.setHasAlpha(true);
        }
    }
Reads next variable length block from input.

@return number of bytes stored in "buffer".
core/src/main/java/com/bumptech/glide/gifdecoder/GifDecoder.java