FFMPEG Plugin: Some cleanups.

- Use name that correctly reflects the return value of avcodec_decode_video2().
- Make DO_PROFILING code path of AVCodecDecoder compile again.
- No functional change intended.
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.
FFMPEG Plugin: Rename some variables in audio path.

- Make the difference between fDecodedData and fDecodedDataBuffer more clear.

- No functional change intended.
FFMPEG Plugin: Refactor scope of fTempPacket in audio path.

- There are two main reasons for this refactoring:
    1. Prepare using FFMPEGs functionality of audio frame start time assignment
       (instead of rolling it ourself) like already done for the video path
       (see _LoadNextVideoChunkIfNeededAndAssignStartTime() for reference).
    2. Get rid of fChunkBufferOffset (this is a minor reason though).

- Untangle some of the conditional checks to increase readability.

- No functional change intended.