title: CV_8UC4 convertTo CV_8UC1 doesn't work
body: <p>I got image from kinect v2 with type CV_8UC4.
  Now I try to use in in type CV_8UC1 with the following code.</p>
  
  <pre><code>cv::Mat mImg(iHeight, iWidth, CV_8UC4);
  cv::Mat mImg2(iHeight, iWidth, CV_8UC1);
  ....
  get image from kinect to mImg
  ....
  mImg.convertTo(mImg2, CV_8UC1);
  </code></pre>
  
  <p>After that, mImg2 type is still 24 ( CV_8UC4 ), not sure whether I use this function correctly. Please help.</p>