FFMPEG Plugin: Fix video start_time handling

- I misinterpreted the semantics of reordered_opaque. I thought it would
  establish the correct relationship between the start_time returned by
  GetNextChunk() and the next video frame successfully decoded. But for this
  to work reordered_opaque expects to be filled with presentation time stamps.
  A series of presentation time stamps may be jumping back in time due to the
  presence of B-frames. The decoded frame presentation time stamps series would
  then be ordered in a monotonically increased way.
  But actually GetNextChunk() always returns monotonically increasing start
  times. Mapping this behaviour to FFMPEG's expectations means labeling those
  start times as decoding time stamps (dts). Though for those start times to be
  related to the correct decoded video frames you have to assign the start time
  with the AVPacket containing the data to be decoded.

- This commit finally makes DVB video playback working for me with the TV app.
  Though no audio yet.

- The documentation was updated accordingly.