Set AVCodecContext.request_channel_layout when decoding.

Allows extracting a 5.1 or 2-channel downmix from a dedicated substream (TrueHD).

Also works around a bug when decoding TrueHD Mono.



git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5242 b64f7644-9d1e-0410-96f1-a4d463321fa5