title: from std::vector<std::vector<cv::Point>> to cv::Mat
body: <p>I want to get every vector of points into a matrix like :</p>
  
  <pre><code>std::vector&lt;std::vector&lt;cv::Point&gt;&gt; vec;
  ......................
  for (int i ; i &lt; vec.size();i++){
     imshow("stuff", cv::Mat(vec[i]); /// this crashes !!!
  }
  </code></pre>
  
  <p>any idea how to do that? 
  thanks in advance </p>