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