Rate control adjustment.

Small adjustment to the active Q range calculations.
These changes should slightly extend the available Q range
for KF/GF/ARF and narrow it for other frames.

The results for this change in isolation are broadly positive
for SSIM and average PSNR and slightly up but mixed for opsnr.

derf +0.293% opsnr, +1.286% SSIM
std-hd + 0.528% opsnr, + 1.746% SSIM
yt +0.056% opsnr, +0.457% SSIM
yt-hd -0.147% opsnr, + 0.226% SSIM

Change-Id: If065280342027ecc5d44b49fc1d440dfef041002