title: AVPlayerItem's stepByCount only smooth for forward, but choppy for backwards?
body: <p>I have implemented AVPlayerItem's stepByCount method to manually go through a video frame by frame. Here's how it looks like for forward 1 step.</p>
  
  <pre><code>    AVPlayer *player = [AVPlayer playerWithURL:url];
      [player.currentItem stepByCount:1];
  </code></pre>
  
  <p>And backward 1 step</p>
  
  <pre><code>    AVPlayer *player = [AVPlayer playerWithURL:url];
      [player.currentItem stepByCount:-1];
  </code></pre>
  
  <p>The forward 1 step (going forward in time frame by frame) works well. However when I try to go backward frame by frame, it's not as smooth as the forward step. Am I missing something? Or is this because of the way videos are encoded--it's meant to be viewed forward but not backwards--inherently?</p>