libvpx

Fixes for key frame coding at speed 5.

Keep the same transform cutoff and partition selection
for speed 5 as in speeds >=6 (non-rd speed settings).

Existing setting for key frame at speed 5 allowed transform size
up to 32x32 on key frames, and did not allow for 4x4 block partition size.
This created more visual artifacts on first few frames.

avgPSNR/overallPSNR/SSIM gains of 0.2/0.7/0.8 for rtc_derf(low-res) set,
and 0/0.7/1.1 gains for rtc set.

Change-Id: I8c139ec6c9bb74e14b4ffbad5f12e94f18a59c0b