Various multi-reel fixes.

Various multi-reel fixes.

* _video_frames_enqueued -> _position
* some int -> Frame replacements
* Call Writer::ref_write when there is nothing to encode / write
  for one frame.
* Assume Encoder::encode is called once per output video frame.
* Possibly correct some off-by-1 overlap bugs.
* Don't reset _last_written on FAKE write as I believe there is no need.