title: User custom private data in AVPacket
body: <p>Is there a way to attach some user private data to <code>AVPacket</code> before decoding to be able to match input <code>AVPacket</code> and decoded output <code>AVFrame</code>? Some sort of <code>AVFrame::opaque</code>.</p>
  
  <p>Specifically, decoding process of h264 codestream can do reorder in a case B-frames are present and I would like to identify which <code>AVPacket</code> was decoded to which <code>AVFrame</code>.</p>