title: Ranges without video in AVMutableCompositionTrack and AVMutableVideoComposition issues
body: <p>I create a composition with several video clips, i also add some empty ranges where I show title screens with CoreAnimation.</p>
  
  <pre><code>|--Video1--|--NoVideo(AVsyncLayer)--|--Video2--|
  </code></pre>
  
  <p>Etc... Which all works fine, the problem comes when I start using:
  AVMutableVideoComposition, AVMutableVideoCompositionInstruction and AVMutableVideoCompositionLayerInstruction which I have to do to rotate videos properly.</p>
  
  <pre><code>|--Video1--|--NoVideo(AVsyncLayer)--|
  </code></pre>
  
  <p>Works fine in my AVPlayer, but if I instead do:</p>
  
  <pre><code>|--NoVideo(AVsyncLayer)--|--Video1--|
  </code></pre>
  
  <p>The application crashes the next time I push a new view controller, or pop the current view controller in my Navigation Controller stack. Sometimes with a memory warning sometimes without it, but according to instruments I only allocated 1.5Mb.</p>
  
  <p>I've watched the Working With Media in AVFoundation from last years WWDC several times, and the only thing I can think of is that the comment</p>
  
  <blockquote>
    <p>AVMutableVideoCompositionInstruction must not overlap or contain gaps.</p>
  </blockquote>
  
  <p>Which it really doesn't. I have instructions for the full duration of the composition. But there are gaps in the AVMutableCompositionTrack (my video track) where I don't plan to show video, but only the CoreAnimation layer.</p>
  
  <p>Have anyone else experienced similar issues?</p>