anchor.diff:    }
   
     void _writeGraphicsCtrlExt() {
  -    output!.writeByte(EXTENSION_RECORD_TYPE);
  -    output!.writeByte(GRAPHIC_CONTROL_EXT);
  +    output!.writeByte(_extensionRecordType);
  +    output!.writeByte(_graphicControlExt);
       output!.writeByte(4); // data block size
   
       const transparency = 0;
  
positive.code: 
      _addImage(_lastImage, _width, _height, _lastColorMap!.colorMap, 256);
  
      output!.writeByte(TERMINATE_RECORD_TYPE);
  
      _lastImage = null;
      _lastColorMap = null;
  
negative.code:   void _addImage(Uint8List? image, int width, int height, Uint8List colorMap,
        int numColors) {
      // Image desc
      output!.writeByte(_imageDescRecordType);
      output!.writeUint16(0); // image position x,y = 0,0
      output!.writeUint16(0);
      output!.writeUint16(width); // image size
  