manicure: support new mixdowns added before 0.9.9.

Also, use "none" as the "None" mixdown; this is the name expected by libhb.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5552 b64f7644-9d1e-0410-96f1-a4d463321fa5
hb_mixdown_*: use meaningful values instead of -1.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5553 b64f7644-9d1e-0410-96f1-a4d463321fa5