libav: fix decoding of mpeg4 in transport streams

There was a chicken & egg problem in the initialization sequence.  mpeg4 only
worked in non-transport streams because we use libav for demuxing these.
The initializion of libav demux causes an additional initialization of the
codec which hides the problem.


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6272 b64f7644-9d1e-0410-96f1-a4d463321fa5