Rate control fix.

This fix addresses some problems with very complex clips like
handling of flashes on clips like crew (which was made worse
by an earlier patch (derf and std-hd)).

Most clips a small effect but some between 1 & 2%

Derf +0.039, +0.211%
YT +0.042, +0.083%

Change-Id: I65fc7c13afc31482040068544dd65b8808f5cb4a