private boolean isNetscape() {
    if (block.length < NETSCAPE.length) {
      return false;
    }

    for (int i = 0, N = NETSCAPE.length; i < N; i++) {
      if (NETSCAPE[i] != (char) block[i]) {
        return false;
      }
    }
    return true;
  }
@param is InputStream to decode
@param os OutputStream to write fixed version of gif, if needed. (optional)
animated-gif-lite/src/main/java/com/facebook/animated/giflite/decoder/GifMetadataDecoder.java