decodeIsolate function
Implementation
void decodeIsolate(DecodeParam param) {
  if (param.bytes == null) {
    param.sendPort?.send(null);
    return;
  }
  // Read an image from file (webp in this case).
  // decodeImage will identify the format of the image and use the appropriate
  // decoder.
  final image2 = imageProcess(param);
  param.sendPort?.send(image2);
}