title: Merge audio with video perfectly
body: <p>I've been trying to merge audio with video using <code>AVMutableComposition</code> and <code>AVExportSession</code>. Everything works perfectly except that the audio and video sources don't have the same duration. </p>
  
  <p>So the exported movie is a bit laggy. Is there anyway to resize or redefine the rate of the video so that its duration becomes exactly equal to the audio's duration? For example, if the audio lasts 10 seconds and the video lasts 9 seconds, I'd like to play the video back at 9/10 speed, so they both end at the same time.
  b</p>