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
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