QualityPSNR.create constructor
Create an object which calculates quality.
Implementation
factory QualityPSNR.create(Mat img, {double maxPixelValue = 255}) {
  final p = calloc<cvg.QualityPSNR>();
  cvRun(() => ccontrib.cv_quality_QualityPSNR_create(img.ref, maxPixelValue, p, ffi.nullptr));
  return QualityPSNR.fromPointer(p);
}