vp9: Increase thr_var for 32x32 blocks in var-based partitioning.
vp9: Increase thr_var for 32x32 blocks in var-based partitioning.

For real-time mode, increase variance threshold for 32x32 blocks in
var-based partitioning for resolution >= 720p, so that it is more
likely to stay at 32x32 for high resolution which accelerates the
encoding speed with little/no PSNR drop.

PSNR effect on different speed settings:
speed 8 rtc: 0.02 overall PSNR drop, 0.285% SSIM drop
speed 7 rtc: 0.196% overall PSNR increase, 0.066% SSIM increase
speed 5 rtc_derf: no effect.

Speed up:
gips_motion_WHD, 1mbps: 2.5% faster on speed 7, 2.6% faster on speed8
gips_stat_WHD, 1mbps: 4.6% faster on speed 7, 5.6% faster on speed8

Change-Id: Ie7c33c4d2dd7d09294917e031357fc5476c3a4bb

Topopiccione/libvpx,mwgoldsmith/libvpx,mwgoldsmith/vpx,mwgoldsmith/vpx,mwgoldsmith/libvpx,Topopiccione/libvpx,mwgoldsmith/vpx,ShiftMediaProject/libvpx,Topopiccione/libvpx,mwgoldsmith/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,mwgoldsmith/vpx,ShiftMediaProject/libvpx,Topopiccione/libvpx,mwgoldsmith/libvpx,webmproject/libvpx,Topopiccione/libvpx,mwgoldsmith/libvpx,ShiftMediaProject/libvpx,mwgoldsmith/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,mwgoldsmith/vpx,webmproject/libvpx,webmproject/libvpx,mwgoldsmith/vpx,Topopiccione/libvpx,webmproject/libvpx