title: C++ opencv How to expand Mat dims
body: <p>My <code>tensorflow</code> model expects images to have shape: <code>[1, 1080, 1920, 3]</code> and i'm trying to reshape frame that i captured from video.</p>
  
  <p>It seems easy to do it using python and numpy: <code>np.expand_dims(frame, axis=0)</code>
  But I have no idea how to do it on c++ using <code>Mat</code> from <code>opencv</code>.</p>
  
  <p>Any ideas?</p>