title: opencv multi channel element access
body: <p>I'm trying to learn how to use OpenCV's new C++ interface.</p>
  <p>How do I access elements of a multi channel matrix? For example:</p>
  <pre><code>Mat myMat(size(3, 3), CV_32FC2);
  
  for (int i = 0; i &lt; 3; ++i)
  {
      for (int j = 0; j &lt; 3; ++j)
      {
          //myMat_at_(i,j) = (i,j);
      }
  }
  </code></pre>
  <p>What is the easiest way to do this? Something like cvSet2D of the old interface.<br />
  What is the most efficient way? Similar to using direct pointers in the old interface.</p>