libhb: improve CC detection
We are failing to detect CCs in some sources because the CC data is not
present on every frame.  Makes scan always decode a few extra frames to
give the decode a better chance of seeing the CC data.


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6471 b64f7644-9d1e-0410-96f1-a4d463321fa5