libhb: Allow control of audio mix normalization

libhb: Allow control of audio mix normalization
Since switching to libavresample for audio mixing, our output
volume levels have been reduced because libavresample
does mix level normalization by default.  This change
applies a patch to libav to allow us to disable this behavior
and adds a new field to hb_audio_config_t to allow the
hb frontends to control this feature.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@4884 b64f7644-9d1e-0410-96f1-a4d463321fa5