Various fixes for ffmpeg input files (mp4, avi, mkv, etc.):

Various fixes for ffmpeg input files (mp4, avi, mkv, etc.):
 - always use source time stamps (HB vfr mpeg4 files now handled correctly)
 - handle Microsoft's braindead "VFW packed b-frames"
 - compute average video frame rate from total duration & number of frames (ignore ffmpeg's frame rate which is closer to a max f.r. for vfr video).
 - workaround an ffmpeg audio decode abort when it used SSE instructions on unaligned buffers.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@1687 b64f7644-9d1e-0410-96f1-a4d463321fa5