title: How to convert list to cv::UMat?
body: <p>I have the list, this list gets from dlib detector:</p>
  
  <pre><code>dets = detector(frame, 0)
  dets = [list(rect_to_bb(dets[0]))]
  print(dets)
  [[277, 294, 150, 149]]
  </code></pre>
  
  <p>How I can convert this list to Umat ?</p>
  
  <pre><code>faces = cascade.detectMultiScale(gray, 1.3, 5)
  print(faces)
  [[259 264 182 182]]
  </code></pre>
  
  <p>The exception, when I use the list</p>
  
  <pre><code>    _, landmarks = facemark.fit(gray, dets)
  TypeError: Expected cv::UMat for argument 'faces'
  </code></pre>