Add an additional restriction to bit/complexity based
segmentation based on spatial variance.

Only lower Q when both the number of bits spent
in the initial encoding pass and the spatial complexity are
below a threshold. This will prevent the low Q segments
being used just because there is a surfeit of bits.

Small metrics gains especially opsnr.
derf ~0.2% std-hd ~0.3%

Change-Id: I6a8496d466d673f9b0e2b2ca6304ea7b6d8e1cce