FFMPEG Plugin: Fix video start_time generation.

- Ensure that start times are increased monotonically for video formats that
  contain B-frames, too, as expected by the BMediaDecoders.
  Previously start times were returned that seemed to go back in time for
  videos containing B-frames.
  Tested with resolutionchange.mpg (\see http://samples.ffmpeg.org/MPEG2)
  Note: Even though start times aren't going back in time anymore there are
  times where two consecutive start times are equal. This would need more
  research once this exposes a bug in a real application. Further more this
  might seem like a new bug, but before this commit the equal start times would
  have simply some different start time[s] in between. So at most this is the
  same bug just wearing new clothes :)

- Documentation updated accordingly.