libhb: fix CC roll-ups that move around the screen

These were causing errors in the parser.  Handle them according to 
recommendations in the CC spec.


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6453 b64f7644-9d1e-0410-96f1-a4d463321fa5
libhb: fix blinking of rendered closed captions

Rounding errors in timestamps caused single frame duration gaps


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6454 b64f7644-9d1e-0410-96f1-a4d463321fa5