QSV: ExtBRC/MBBRC and lookahead are mutually exclusive, plus miscelleanous stuff found in testing.

Thanks to gmb for pointing it out, and Nina Kurina for pointing out the ExtBRC/AsyncDepth issue.
 


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5808 b64f7644-9d1e-0410-96f1-a4d463321fa5