title: merge two images with alpha channel
body: <p>I have two images, one with and other without alpha channel. Thus, image <code>A</code> and <code>B</code> has a shape of <em>(x,y,4)</em> and <em>(x,y,3)</em> respectively. </p>
  
  <p>I want to merge both images in a single tensor using <code>python</code>, where <code>B</code> is the background and <code>A</code> is the upper image. The final image must have a shape of (x, y, 3). I tried if scikit-image or cv2 is capable of doing this, but I couldn't found any solution.</p>