Handle boundaries between blueray clips better.

Handle boundaries between blueray clips better.

These boundaries are always discontinuities.  But sometimes we were
not detecting them as such and would drop frames.  So set a flag
in the buffer when libbluray tells us a new clip is starting and
use that to trigger computation of a new scr offset.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@3912 b64f7644-9d1e-0410-96f1-a4d463321fa5