libvpx

Adjust active maxq for GF groups.

Currently disabled by default: enabled using
#define GROUP_ADAPTIVE_MAXQ

In this patch the active max Q is adjusted for each GF
group based on the vbr bit allocation and raw first pass
group error.

This will tend to give a lower q for easy sections
and a higher value for very hard sections. As such it is
expected to improve quality in some of the easier
sections where quality issues have been reported.

This change tends to hurt overall psnr but help
average psnr. SSIM also shows a small gain.

Average results for derf, yt, std-hd and yt-hd test sets were
as follows (%change for average psnr, overal psnr and ssim):-

derf +0.291, - 0.252, -0.021
yt +6.466, -1.436, +0.552
std-hd +0.490, +0.014, +0.380
yt-hd +5.565, - 1.573, +0.099

Change-Id: Icc015499cebbf2a45054a05e8e31f3dfb43f944a