The nvidia test suite was never run after livepeer#393 so this breakage
was not noticed.

* Set the encoder timebase using AVCodecContext.framerate instead of
  the decoder's AVCodecContext.time_base.

  The use of AVCodecContext.time_base is deprecated for decoding.
  See https://ffmpeg.org/doxygen/3.3/structAVCodecContext.html#ab7bfeb9fa5840aac090e2b0bd0ef7589

* Adjust the packet timebase as necessary for FPS pass through
  to match the encoder's expected timebase. For filtergraphs using
  FPS adjustment, the filtergraph output timebase will match the
  framerate (1 / framerate) and the encoder is configured for the same.

  However, for FPS pass through, the filtergraph's output timebase
  will match the input timebase (since there is no FPS adjustment)
  while the encoder uses the timebase detected from the decoder's
  framerate. Since the input timebase does not typically match the FPS
  (eg 90khz for mpegts vs 30fps), we need to adjust the packet timestamps
  (in container timebase) to the encoder's expected timebase.

* For the specific case of FPS passthrough, preserve the original PTS
  as much as possible since we are trying to re-encode existing frames
  one-to-one. Use the opaque field for this, since it is already being
  populated with the original PTS to detect sentinel packets
  during flushing.

  Without this, timestamps can be slightly "squashed" down when
  rescaling output packets to the muxer's timebase, due to the loss of
  precision (eg, demuxer 90khz -> encoder 30hz -> muxer 90khz)