Interop: Fixed a bug that was causing crashes on AAC passthrough. We were clobbering the private data section with zeroes. Also added support for passing through a track if it's possible.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6210 b64f7644-9d1e-0410-96f1-a4d463321fa5