title: iOS8 UINavigationController push/pop with transparent UIView issue
body: <p>I noticed in iOS8, when I'm pushing a new UIViewController in a UINavigationController, the old UIViewController slides out but is visible in my new top foreground UIViewController if it has a non opaque background.</p>
  
  <p>When the push is done, the old controller just disappears, leaving the whole animation looking rough. </p>
  
  <p>Previous to iOS8 you would not see the view behind the one getting pushed in even if it was transparent. As if there was a mask that slid in as well. Anyone know of a good way to fix this so it's like the old behavior without re-implementing the entire push animation with a custom animation?</p>
  
  <p><img src="https://i.stack.imgur.com/CB8cw.png" alt="enter image description here"></p>