Minor amendments to 5.1 audio support:

Minor amendments to 5.1 audio support:

* ensuring that non-AAC audio formats always use a stereo mix (rather than mono)
* ensuring that Dolby Surround (Pro Logic) source is passed through as Dolby Surround (Pro Logic), and not mixed down to Stereo
* storing the ac3 flags with the job for later reference

Note: this is still using the settings for the last selected soundtrack for *all* soundtracks.  This needs improving, as they may not be of the same format!

Tested with mono, stereo, pro logic and 5.1 AC3 sources, to all output audio formats, with success all round :)

git-svn-id: svn://svn.handbrake.fr/HandBrake/branches/0.8.0_beta2_5.1@319 b64f7644-9d1e-0410-96f1-a4d463321fa5