private Bitmap getNextBitmap() {
        Bitmap result = bitmapProvider.obtain(header.width, header.height, BITMAP_CONFIG);
        if (result == null) {
            result = Bitmap.createBitmap(header.width, header.height, BITMAP_CONFIG);
        }
        setAlpha(result);
        return result;
    }
Reads next variable length block from input.

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