title: OpenCV draw an image over another image
body: <p>Is there an OpenCV function to draw an image over another image?
  I have one big image of <code>Mat</code> type.
  And I have a small image of <code>Mat</code> type (<code>5x7</code>).
  I want to draw this small image over the big image at specified <code>coordinates</code>.</p>