Account for rate error in GF group Q calculation.
When GF group adaptive maxQ is enabled this patch accounts somewhat for accumulated error in the rate control. This improves accuracy quite a bit on many clips especially when there is overshoot. Examples when the overshoot and undershoot command line parameters are set to 100: Hall @ 1200 overshoot is reduced from 67-24%. Akiyo @ 400 undershoot is reduced from 28%-15%. Setting a lower value for undershoot or overshoot still reduces the error further. Impact on metrics is mixed with some gains in average psnr but generally a little lower (e.g. 0.5%) on overall and ssim. The GF group adaptation is still off by default in this patch. Compared to with the head, enabling this mode now gives big average psnr gains on the YT sets (e.g. YT_HD >11.2%), a drop in overall PSNR (YT-HD 3.9%) and a smaller drop or neutral for SSIM. Change-Id: If4b32cd0740d3fb941317b374f9c2951954eee90