title: change avplayer play rate and then save video as a movie file
body: <p>I have a video which I play by using avplayer.
  And then I have a feature where user can change rate of the movie to go faster or slower. Is there a way to say this fast or slowed down movie?</p>
  
  <p>I have looked into avassetwriter but I can't find a way to pass in a rate prperty.</p>