private GifHeaderParser getHeaderParser() {
        if (parser == null) {
            parser = new GifHeaderParser();
        }
        return parser;
    }
Reads GIF image from stream.

@param is containing GIF file.
@return read status code (0 = no errors).
core/src/main/java/com/bumptech/glide/gifdecoder/GifDecoder.java