title: canvas.toDataURL() returns a black image
body: <p>We are developing a sample three.js application which shows a thumbnail.
  For any scene that we create, we require to take a screenshot of the canvas,
  for this purpose, we are using canvas.toDataURL("image/png").
  For repeated invocations of the above function, it returns the same black image,
  despite elements being added to the canvas.</p>
  
  <p>We are using WebGLRenderer and threejs r99</p>
  
  <p><a href="https://i.stack.imgur.com/hB5ve.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/hB5ve.png" alt="Live Canvas"></a>
  <a href="https://i.stack.imgur.com/khVpE.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/khVpE.png" alt="canvas.toDataURL()"></a></p>
  
  <p>I am using google chrome and threejs. Since the canvas context will be made by threejs itself, I cannot add that parameter (preserveDrawingBuffer) shown in the question <a href="https://stackoverflow.com/questions/26783586/canvas-todataurl-returns-blank-image-only-in-firefox">Canvas toDataURL() returns blank image only in Firefox</a></p>