This change subtracts out low complexity intra regions that are also low
error in the inter domain, in the calculation of the frame prediction decay.
The rationale here his that low complexity regions (such as sky) do not imply
high prediction decay in the same way as high error intra or neutral blocks.

The effect of this is small in most clips but in a few clips it can be > 10%.
(E.g. In to tree)

Change-Id: If67ac23d17fca14285cad2defa464c61c9ea861c