libvpx

Allow sub8x8 intra modes test for alt frame coding

This commit allows sub8x8 intra modes test in the rate-distortion
loop for hd sequences in speed 1 and 2.

For sequence y90n of hd set at 8000 kbps, speed 2 runtime goes
from 207s to 210s. For ped_1080p at 3000 kbps, speed 2 runtim goes
from 336s to 337s. Both are running with 300 frames.

This improves compression performance by 0.24% for stdhd and 0.32%
for hd.

Change-Id: I173ca38a6411565ae6cfadd184c42b2070c5de1f