Makes the delta-qp stop little earlier on areas that have been refreshed enough.
This helps to reduce some pulsing artifact on noisy flat areas observed in some
noisy vc-clips.

Threshold changes only take effect for sources where noise level is estimated to
be >= medium level.

Only affects 1 pass CBR, non-screen content case.

Change-Id: Iacf557f6aa8abbcd6782c02ff2e6c14891960850