FFMPEG-Plugin: Refactor out update of media_header.

- Main purpose is to make reading the function DecodeNextFrame() easier on the
  eyes, by moving out auxiliary code.
  Note: The media_header update code for the start_time is still left in
  DecodeNextFrame(). This will be addressed in a later commit specially
  targetted on handling start_time calculations for incomplete video frames.

- Also updated / added some documentation.

- No functional change intended.

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