FFMPEG Plugin: Refactor audio decoding method into two.

- First method is solely responsible to fill the audio output buffer with
  already decoded audio frames.
  Second method is solely responsible for decoding the encoded audio data and
  put it in the decoded audio output buffer for further processing with the
  first method.
  This prepares auto detection of audio frame properties for audio formats
  where the properties are contained within the encoded audio frame (e.g. MP3),
  instead within the audio container format (e.g. WMA). Implementing auto
  detection is scheduled for a later commit though.

- Added documentation accordingly.

- No functional change intended.