Modify variance partition selection for low resolutions.

For low spatial resolutions: bias partittion selection to smaller block sizes,
and base the variance computation on 4x4 down-sampling.

Also move the threshold computations into the choose_partitioning,
so they are computed once for each sb block.

On low-res clips (RTC_derf) PSNR/SSIMetrics increase by about 4-5%.
No change for resolutions above CIF.

Change-Id: I93f8ff742c8044786977bb6e31dcf8efda6dd1b0