Fix a problem with overlapping vobsubs

Fix a problem with overlapping vobsubs
Since we now allow subtitles that overlap in time, it is no longer
appropriate to arbitrarily set the duration to 3 seconds when vobsubs
don't have an explicit stop time.  This causes them to overlap on the display.
So now, we set the stop time for such vobsubs to -1. Then in sync adjust the
stop time to the start of the next vobsub when it is seen.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@3961 b64f7644-9d1e-0410-96f1-a4d463321fa5