title: How can i detect whether two CALayers overlap?
body: <p>In my app the user can drag the top CALayer from a stack of CALayers.</p>
  
  <p>When the users stops dragging the CALayer should return to its original position on top of the others CALayers. Unless the CALayer (after dragging) no longer overlaps with the other CALayers in which case it should move underneath the other CALayers.</p>
  
  <p>Now, how can I detect whether two CALayers overlap? </p>