libhb: fix setting thread_count for libav encoding


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6389 b64f7644-9d1e-0410-96f1-a4d463321fa5
LinGui: enable "More Settings" box in video tab for VP8


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6390 b64f7644-9d1e-0410-96f1-a4d463321fa5