title: python OpenCV - add alpha channel to RGB image
body: <p>What is the best way to convert RGB image to RGBA in python using opencv?</p>
  
  <p>Let's say I have one array with shape</p>
  
  <pre><code>(185, 198, 3) - it is RGB
  </code></pre>
  
  <p>and the other is alpha mask with shape <code>(185, 198)</code></p>
  
  <p>How to merge them and save to file?</p>