libvpx

Enable group adaptive max q by default.

Set the GF group adaptive max Q compile flag to 1 by default.

This change has a quite big visual impact in some clips and also
contributes to tighter rate control.

For short test clips that have consistent content the impact is
quite small on metrics but for more varied long form clips there is
a drop in overal psnr but a sharp rise in average psnr caused by
greater expenditure on some easier sections and tighter rate clipping
in hard sections.

In chunck'ed encodes some of the effect will already be present due
to the independent rate control in each chunk but this change takes
the control down to a smaller scale.

yt hd +10.67%, - 3.77%, -1.56%
yt +9.654%, - 3.6%, - 1.82%
std hd +0.25%, -0.85%, -0.42%
derf +0.25%, - 1.1%. - 0.87%

Change-Id: Ibbc39b800d99d053939f4c6712d715124082843e