title: AVAssetExportSession using AVAssetExportPresetPassthrough breaking output
body: <p>I'm using AVAssetExportSession in combination with AVAssetExportPresetPassthrough to stitch multiple videos together. Everything works quite fine, except after my first sub-clip should have finished, it's picture "freezes" on the last frame but the second is not going to play. I made sure to set the layer opacity to 0.0f once each clip has finished, if I use another Preset-Type everything works...
  Any hints?</p>