- Re-use our audio decoder modules instead of using libavcodec directly.
  - No more dependance on libavcodec.
  (downmixing is currently broken + you have to force the ffmpeg codec for a52)
* modules/audio_filter/format.c:
  - PCM audio format conversion filter using the new common filter architecture.
* modules/audio_filter/converter/mpgatofixed32.c:
  - implements both the old and new filter architecture (ie. useable in the transcoder).
* modules/codec/ffmpeg/audio.c:
  - fixes and cleanup.