libvpx

Fix rate control bug.

Fix rate control bug whereby the rate factor heuristics
were being updated on arf overlays causing a rate surge
for a few frames followed by a corrective drop.

This fix eliminates many of the overshoot problems that
we were seeing on hard clips (even without applying
stricter vbr rate control) and also helps quality on
almost all clips with some hard clips improving by >5%.

Overall quality results measured at speed 2.
Derf  +1.78% opsnr , +2.44% SSIM
Stdhd +2.41% opsnr, +2.85% SSIM

Change-Id: I2369df6295c2705963fa6307877f6acb304bcc39