libvpx: allow 0 as min quantizer value

Allow setting the min quantizer to 0 instead of 1 (libvpx allows 0); fixes #2136

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
libvpx: check if CQ level is in correct bounds

Check whether CQ level is within qmin..qmax and abort on error. If this isn't properly
set, encoding will fail at the first frame.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>