title: Finding the size in bytes of cv::Mat
body: <p>I'm using OpenCV with <code>cv::Mat</code> objects, and I need to know the number of bytes that my matrix occupies in order to pass it to a low-level C API. It seems that OpenCV's API doesn't have a method that returns the number of bytes a matrix uses, and I only have a raw <code>uchar *data</code> public member with no member that contains its actual size.</p>
  
  <p>How can one find a <code>cv::Mat</code> size in bytes?</p>