Speed up encoding by skipping altref recode

Speed up encoding by skipping altref recode

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