libvpx

Adapt strength of AQ2.

Adapt the use of segmentation in AQ mode 2 based on
the ambient kf/arf/gf Q.

Disable segmentation where the rate per SB is very
low and overheads are likely to outweigh the benefits.

This patch reduces the -ve average metrics impact
of AQ mode 2 while allowing stronger 3 segment AQ
in some cases. Average improvement ~0.5-1.0%.

Change-Id: I5892dfcc7507c5cc6444531cc7fe17554cf8d0c7