title: Cross-fade within AVMutableVideoComposition
body: <p>I have successfully composed an AVMutableComposition with multiple video clips and can view it and export it, and I would like to be able to transition between them using a cross-fade, so I want to use AVMutableVideoComposition.  I can't find any examples on how to even arrange and play a couple AVAsset videos in succession.  Does anyone have an example of how to add tracks to an AVMutableVideoComposition with the equivalent of AVMutableComposition's insertTimeRange, or how to set up a cross-fade?</p>
  
  <pre><code>[self.composition insertTimeRange:CMTimeRangeMake(kCMTimeZero,asset.avAsset.duration)
                                           ofAsset:asset.avAsset
                                            atTime:self.composition.frameDuration
                                             error:nil]
  </code></pre>