vp9: Speed 8: More aggresive golden skip for low res.

vp9: Speed 8: More aggresive golden skip for low res.

Add a new, more aggresive short circuit: short_circuit_low_temp_var = 3 to skip
golden of any mode when variance is lower than threshold for low res.
This change only affects speed = 8, low resolution.

Metrics for avgPSNR/SSIM on rtc_derf (low resolution) show loss of
0.27/0.31%.
On Nexus 6, the encoding time is reduced by ~2.3% on average across all
low-res clips.

Visually little change on rtc_derf clips.

Change-Id: Ia8f7366fc2d49181a96733a380b4dbd7390246ec