FFMPEG Plugin: Implement audio input buffer padding.

- Padding is required by FFMPEG for correct operation of all audio decoders.
  FFMPEG performs some speed optimizations under the hood that may lead to
  reading over the end of the chunk buffer wouldn't there have been padding
  applied.

- Resolve TODOs by unifying fVideoChunkBuffer and fChunkBuffer back into
  fChunkBuffer because audio path is responsible for freeing fChunkBuffer now.
  Resolved TODOs apply to the replacing fVideoChunkBuffer variable by
  fChunkBuffer, rename some methods by removing the "Video" part, collapse two
  methods into one (_LoadNextChunkIfNeededAndAssignStartTime()).
  No functional change intended.

- Update documentation accordingly.