title: Masking camera preview on a TextureView
body: <p>I'm using a TextureView to show the camera preview. For project needs it should be square and with rounded corners. This is working fine for the square part, as I can use the transformation Matrix to readjust the ratio deformation, but I have no clue on how to mask the video with rounded corners.</p>
  
  <p>Basically what I've found is:</p>
  
  <ol>
  <li>As the TextureView is showing the camera preview, I cant' lock the canvas to modify it.</li>
  <li>I can't override onDraw as it's declared final in the TextureView class (it makes quite sense)</li>
  </ol>
  
  <p>Is there any way to process the camera output out of the transformation Matrix?</p>