Add adaptation option for VBR.
Allow min and maxQ to creep when the undershoot or overshoot exceeds thresholds controlled by the command line under_shoot_pct and over_shoot_pct values. Default is 100%,100% which ~disables adaptation. Derf results for example undershoot% / overshoot%:- Head:- Mean abs (%rate error) = 14.4% This check in:- 25%/25% - Mean abs (%rate error) = 6.7% PSNR hit -1% SSIM -0.1% 5% / 5% - Mean abs (%rate error) = 2.2% PSNR hit -3.3% SSIM - 1.1% Most of the remaining error and most of the quality hit is at extreme data rates. The adaptation code still has an exception for material that is in effect static so that we don't over adjust and over spend on YT slide show type content. (Rebase of If25a2449a415449c150acff23df713e9598d64c9 to resolve a auto-merge error) Change-Id: Iec4e1613ef0d067454751d8220edb7058dfbd816