title: How to convert opencv Matx to Mat
body: <p>I'm looking to convert a <code>cv::Matx44d</code> to a <code>cv::Mat</code> of type <code>double</code> and size <code>rows = cols = 4</code></p>
  <p>I've seen this answer that talks about the reverse: <a href="https://stackoverflow.com/questions/17919814/how-to-convert-cvmat-to-cvmatx33f">how to convert cv::Mat to cv::Matx33f</a></p>
  <p>Thank you</p>