title: Set allocated Mat with raw data OpenCV
body: <p>I have a <code>uchar* raw</code> from an API which represents an image raw data. The width, height and number of channels of this <code>raw</code> is already known. I have already pre-allocated an cv::Mat (OpenCV) with this width and height. </p>
  
  <p>My question is - how is it possible to set raw into this cv::Mat. I would like to copy raw into cv::Mat instead of just switching pointers. Is there a function to accomplish this or I need to do so manually myself?</p>