MacGui: In presets with an audio track specified for pass thru ( either AC3 or DTS...

MacGui: In presets with an audio track specified for pass thru ( either AC3 or DTS ) prevent a mixdown in any track besides track one.
 - If pass thru is specified in any track > track 1in the preset,  set that track to "None" if a matching source codec cannot be found.
- Still allow track 1 to mixdown if a matching source track codec is not present. Prevents accidental encodes with no audio.
- Among other things prevents a duplication of tracks with an aac mixdown for the AppleTV preset when using a source with audio other than AC3.

git-svn-id: svn://localhost/HandBrake/trunk@2876 b64f7644-9d1e-0410-96f1-a4d463321fa5