featureAsync method
Implementation
Future<Mat> featureAsync(Mat alignedImg, {bool clone = false}) async {
  final ret = Mat.empty();
  return cvRunAsync0(
    (callback) => cobjdetect.cv_FaceRecognizerSF_feature(ref, alignedImg.ref, clone, ret.ptr, callback),
    (c) {
      return c.complete(ret);
    },
  );
}