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.

- Enhance "logging stream to file" functionality to write to distinct logging
  files for audio and video. Before this commit one could only log video
  streams. But with unifying the _LoadNextChunkIfNeededAndAssignStartTime()
  audio streams gained the logging functionality for free. But now audio and
  video streams would be written in the same log file when watching a media
  file containing both audio and video. This is prevented by the distinct
  logging mentioned above.

- Update documentation accordingly.