libhb: improve handling of overlapping text subtitles
mp4 timed text does not support subtitles that overlap (in time).
So this patch detects these overlaps and merges subtitles when
appropriate.


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5632 b64f7644-9d1e-0410-96f1-a4d463321fa5