ffmpeg: move A/V non-streamcopy initialization to a later point

ffmpeg: move A/V non-streamcopy initialization to a later point

- For video, this means a single initialization point in do_video_out.
- For audio we unfortunately need to do it in two places just
  before the buffer sink is utilized (if av_buffersink_get_samples
  would still work according to its specification after a call to
  avfilter_graph_request_oldest was made, we could at least remove
  the one in transcode_step).

Other adjustments to make things work:
- As the AVFrame PTS adjustment to encoder time base needs the encoder
  to be initialized, so it is now moved to do_{video,audio}_out,
  right after the encoder has been initialized. Due to this,
  the additional parameter in do_video_out is removed as it is no
  longer necessary.