title: toDataURL throw Uncaught Security exception
body: <p>I have two Set of Code for testing html5 canvas</p>
  
  <p>Set 1  - Work perfectly</p>
  
  <pre><code>&lt;img id="preview" src="http://www.gravatar.com/avatar/0e39d18b89822d1d9871e0d1bc839d06?s=128&amp;d=identicon&amp;r=PG"&gt;
  &lt;canvas id="myCanvas"/&gt;
  
  var c = document.getElementById("myCanvas");
  var ctx = c.getContext("2d");
  var img = document.getElementById("preview");
  ctx.drawImage(img, 10, 10);
  alert(c.toDataURL());
  </code></pre>
  
  <p>Set 2 - Show exception error (Uncaught SecurityError: Failed to execute 'toDataURL' on 'HTMLCanvasElement': tainted canvases may not be exported. )</p>
  
  <pre><code>&lt;img id="preview1" src="http://www.gravatar.com/avatar/0e39d18b89822d1d9871e0d1bc839d06?s=128&amp;d=identicon&amp;r=PG"&gt;
  
  function getBase64() {
      var img = new Image();
      img =  document.getElementById("preview1");
      var canvas = document.createElement("canvas");
      canvas.width =img.width;
      canvas.height =img.width;
      var ctx = canvas.getContext("2d");
      ctx.drawImage(img, 0, 0);
      var dataURL = canvas.toDataURL("image/png"); //This line of code will throw exception
      alert(  dataURL.replace(/^data:image\/(png|jpg);base64,/, "")); 
  }
  </code></pre>
  
  <p>I have no idea why in Set 1 <code>toDataURL</code> is not throwing any exception where Set <code>toDataURL</code> will throw exception and both are using same set of image.The different is in the first set i hardcode the canvas in HTML , and second set i create it through javascript.</p>
  
  <p>My objective for Set 2 code is to get 64 base encode of the image.</p>