FFMPEG Plugin: Fix playing video files.

- This should fix the bug where video files that played well before the recent
  changes to the FFMPEG Plugin didn't play anymore. Now we apply the essential
  video container properties (that were passed by with Setup()) to the
  AVCodecContext. Some video formats simply store those properties in the
  container only (e.g. AVI, WMV) and not in the video frames itself
  (e.g. MPEG2).
  Tested with several files from samples.ffmpeg.org and from the FATE suite of
  FFMPEG.