Speed up for speed 0.
Reduce 10+% of encoding time for hdres in speed 0,
with less than 0.1% PSNR loss.
Compute total difference of previous and current frame context probability
model. If the diff is less than the threshold, skip recoding the frame.
Borg test (positive number means performance loss):
lowres midres hdres
PSNR: 0.030 0.032 0.065
Local speed test: bitrate set at 1200
blue_sky pedestrian rush_hour
Encoding time: -10.0% -16.5% -16.5%
Change-Id: I4e2d200ea3115d48b2c3e890143596b31b8ef9e9
76567d84ce