title: Instascan throws no error but does not show video preview
body: <p>I'm using the <a href="https://github.com/schmich/instascan" rel="nofollow noreferrer">Instascan</a> library in the latest Chrome version (78.0.3904.108). I have a very basic setup with my Html page (<code>index.html</code>):</p>
  
  <pre class="lang-html prettyprint-override"><code>&lt;head&gt;
      &lt;script type="text/javascript" src="https://rawgit.com/schmich/instascan-builds/master/instascan.min.js"&gt;&lt;/script&gt; &lt;!--Qr code scanner--&gt;
      &lt;script type="text/javascript" src="user.js"&gt;&lt;/script&gt;
  &lt;/head&gt;
  &lt;body&gt;
      &lt;p&gt;&lt;button id="btn_start" onclick="start()"&gt;Start&lt;/button&gt;&lt;/p&gt;
      &lt;video id="preview"&gt;&lt;/video&gt;
  &lt;/body&gt;
  </code></pre>
  
  <p>And my javascript (<code>user.js</code>)</p>
  
  <pre class="lang-js prettyprint-override"><code>var scanner = new Instascan.Scanner({ video: document.getElementById('preview') });
  
  function start() {
      Instascan.Camera.getCameras()
          .then((cameras) =&gt; {
              scanner.start(cameras[0]);
          })
  }
  </code></pre>
  
  <p>When I press start, I can see in the debugger that <code>scanner.start</code> is executed with my webcam. No error is shown in the console. However, the video element shows nothing.</p>
  
  <p>How do I make the video element show what my webcam sees?</p>