title: Recording multiple canvas using MediaRecorder
body: <p>I want to record the multiple layered canvas using <strong>MediaRecorder</strong>.<br />
  But, i don't know how to achieve it...
  help me...</p>
  <p>this is the my pseudo code</p>
  <pre class="lang-js prettyprint-override"><code>const RECORD_FRAME = 30;
  
  const canvasVideoTrack = canvas.captureStream(RECORD_FRAME).getVideoTracks()[0];
  const waterMarkCanvasTrack = waterMarkCanvas.captureStream(RECORD_FRAME).getVideoTracks()[0];
  
  const stream= new MediaStream();
  const mediaRecorder = new MediaRecorder(stream);
  
  mediaRecorder.stream.addTrack(canvasVideoTrack)
  mediaRecorder.stream.addTrack(waterMarkCanvasTrack)
  
  // .... recording
  </code></pre>