title: canvas unable to generate bmp image dataurl in chrome
body: <p>I have code to draw image from video. Here is the code</p>
  
  <pre><code>&lt;script type="text/javascript"&gt;
  
  function capture() {
  
      var video  = document.getElementById("videoId");
      var canvas = capture(video, 1);
      var dataURL = canvas.toDataURL("image/bmp", 1.0);
      console.log("dataurl: "+dataURL);
  
  }
  
  &lt;/script&gt;
  
  
  &lt;body&gt;
   &lt;input type="button" value="Capture" onClick="capture()"/&gt;
   &lt;video id="videoId" width="640" height="480"/&gt;
  &lt;/body&gt;
  </code></pre>
  
  <p>on console the dataurl is shown as "data:image/png;base64,..."</p>
  
  <p>Question?</p>
  
  <p>why dataurl is generated in png format??</p>
  
  <p>Note: 
    This has happened in chrome browser[41.0.2272.89].
    In firefox url is generated in bmp format.</p>