Use hb_audio_resample for downmixing AC3 sources. DRC is still applied by liba52.

Use hb_audio_resample for downmixing AC3 sources. DRC is still applied by liba52.

Add support for center & surround mix levels to hb_audio_resample.

This allows us to support upmixing all audio sources. For sources that have at least 2 front and one back or side channel(s), allow upmixing to 5.1:

3.0/3.1 (2 front and 1 back channels)
4.0/4.1 (3 front and 1 back channels)
4.0/4.1 (2 front and 2 side channels)
5.0     (3 front and 2 side channels)

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@4885 b64f7644-9d1e-0410-96f1-a4d463321fa5