title: Render off-screen SCNScene into UIImage
body: <p>How can I render render an <em>off-screen</em> <code>SCNScene</code> into a <code>UIImage</code>? </p>
  
  <p>I know that <code>SCNView</code> provides a <code>-snapshot</code> method, but unfortunately that doesn't work for off-screen views. A <a href="https://stackoverflow.com/questions/23503472/cannot-create-a-screenshot-of-a-scnview?lq=1">similar question</a> have been asked before where one of the answers suggest reading the bitmap data from OpenGL using <code>glReadPixels</code>, but that approach doesn't work for me with an off-screen scene.</p>
  
  <p>I tried rendering into the context of an <code>GLKView</code> using <code>SCNRenderer</code> without success.</p>