message: highgui: Qt - restore convertscale semantics
  broken in
diff: @@ -2557,7 +2557,14 @@ void DefaultViewPort::updateImage(const CvArr* arr)
  nbChannelOriginImage = cvGetElemType(mat);
  CV_Assert(origin == 0);
  cv::Mat src = cv::cvarrToMat(mat), dst = cv::cvarrToMat(image2Draw_mat);
  - cv::cvtColor(src, dst, cv::COLOR_BGR2RGB, dst.channels());
  +
  + cv::Mat tmp;
  + int src_depth = src.depth();
  + double scale = src_depth <= CV_8S ? 1 : src_depth <= CV_32S ? 1./256 : 255;
  + double shift = src_depth == CV_8S || src_depth == CV_16S ? 128 : 0;
  + cv::convertScaleAbs(src, tmp, scale, shift);
  +
  + cv::cvtColor(tmp, dst, cv::COLOR_BGR2RGB, dst.channels());
  CV_Assert(dst.data == image2Draw_mat->data.ptr);
  viewport()->update();
  