title: Exactly what should happen in a CALayer's display/drawRect methods?
body: <p>Which, if either, of these methods would be an appropriate place to:</p>
  
  <ul>
  <li>Change the text of a CATextLayer</li>
  <li>Load a different image into a CAImageLayer</li>
  <li>Tell sublayers to update themselves</li>
  </ul>