FFMPEG-Plugin: Simplify start time calculation of video frame.

- We let FFMPEG keep track of the correct relationship between presentation
  start time of the encoded video frame and the resulting decoded video frame.
  This simplyfies our code, meaning less lines of code to maintain :)

- Update documentation and pointing out some corner cases when calculating the
  correct presentation start time of a decoded video frame under certain
  circumstances.

- Fix doxygen: Use doxygen style instead of javadoc style.

- No functional change intended.

Signed-off-by: Colin Günther <coling@gmx.de>
FFMPEG-Plugin: Fix doxygen style and typo.

- No functional change intended.

Signed-off-by: Colin Günther <coling@gmx.de>