libvpx

Fixes on feature disabling split based on variance

Adds a couple of minor fixes, which may be absorbed in Jingning's
patch. Thanks to Guillaume for pointing these out.
Also adjusts the thresholds for speed 1 and 2 to 16 and 32
respectively, to keep quality drops small.

Results:
--------
derfraw300:  threshold = 16, psnr -0.082%, speedup 2-3%
             threshold = 32, psnr -0.218%, speedup 5-6%
stdhdraw250: threshold = 16, psnr -0.031%, speedup 2-3%
             threshold = 32, psnr -0.273%, speedup 5-6%

Change-Id: I4b11ae8296cca6c2a9f644be7e40de7c423b8330