title: Use both autoclear and preserveDrawingBuffer in WebGLrenderer
body: <p>I have a Three.js scene, where I have set renderer's preserveDrawingBuffer:true because I want to use renderer.toDataURL to take a snapshot of what is on the canvas, and I also use renderer.autoClear = false because I am using 2 scenes with one camera each (one renderer of course).</p>
  
  <p>The problem is that when I move the camera with orbitControls, the objects do not animate as they should, their previous states stay on the canvas and they are creating an ugly painting ( they are not erased) because of the preserveDrawingBuffer:true setting. </p>
  
  <p>What can I do to have both the snapshot functionality the 2 scenes/cameras and the orbit controls?</p>
  
  <p>Thank you.</p>