Limit Q variability in two pass.
In two pass encoding each frame is given an active Q range to work with. This change limits how much this Q range can be altered over time from the initial estimate made for the clip as a whole. There is some danger this could lead to overshoot or undershoot in some corner cases but it helps considerably in regard to clips where either there is a glut or famine of bits in some sections, particularly near the end of a clip. Change-Id: I34fcd1af31d2ee3d5444f93e334645254043026e
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