FFMPEG Plugin: Implement flushing of video frames from decoder.

- Video frames still contained in the video decoder are now flushed when there
  are no encoded data chunks left (signaled by B_LAST_BUFFER_ERROR).
- New code was successfully tested with mpeg2_decoder_test. The last test image
  is now decoded, too, resulting in a bump of the expected number of decoded
  images.
- Refactor code to support flushing -and- keep the readability at a sane level.
- Remove some uncommented code for the sake of readability.
- Documentation updated accordingly.