FFMPEG Plugin: Refactor out loading next video chunk.

- Main reason for refactoring was to increase readability of
  _DecodeNextVideoFrame() by simplifying it. Refactoring was tested
  successfully for no functional change with mpeg2_decoder_test.
- Reindented the method definition in the header file so that the new method
  _LoadNextVideoChunkIfNeededAndUpdateStartTime() fits into 80 chars per line.
  Reindentdation is applied to methods only as the member variables have no
  space left for reindentation.
- Update documentation accordingly.
- Fix wording of audio part to audio path.
- No functional change intended.