If a superblock contains alot of "skin" then force split
of 64x64 partition, and make some adjustments in mode selection.

This helps to reduce artifacts on moving face/skin areas at low bitrates.

Little/no change in metrics: avgPSNR/SSIM down by ~0.12%.
Small encoding time increase < 1%.

Change-Id: Ic57f52148c3716f391419fab0530d916e4c1d186