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