Disconnect ARF breakout from frame boost.

Disconnect ARF breakout from frame boost.

This small change replaces the frame boost check in the arf group
length break out clause with a test against a prediction decay value.

The boost value is in fact partly dependent on the decay value but
this change means that the per frame boost calculation can be adjusted
without influencing the group length calculation.

The value chosen gives a close match on all the test sets with the previous
code (on average) but it was noted that a lower threshold was slightly better
for 1080P and up and a slightly higher value for small image sizes.

Change-Id: I4d5b9f67d5b17b0d99ea3f796d3d6202fd61ee0c