title: AVCaptureVideoPreviewLayer not working with renderInContext
body: <p>I have an <code>AVCaptureVideoPreviewLayer</code> (that works fine showing the video fullscreen) that I am trying to show a small preview in another small view that I have added as a subview.  To do this, I was trying to use <code>renderInContext</code> but all I get is a black screen in my "preview view".</p>
  
  <pre><code>[[self.captureView.layer.sublayers objectAtIndex:0] renderInContext:context];
  </code></pre>
  
  <p>Any other layers seem to work fine and show up in my "preview view", why does <code>AVCaptureVideoPreviewLayer</code> not?</p>