added separate entropy context for alt_ref

This commit added code to keep track of separate entropy contexts for
normal frames and alt ref frames. The underly assumption was that the
two type of frames have different entropy characteristics given they
typically have quite different quantization levels. By keeping entropy
contexts separate, it helps the entropy context distribution to be more
closely adapted to each frame type.

Tests on derf set showed a good and very consistent gain on all clips
on all metrics, avg psnr: 0.89%, overall psnr: 0.84% and ssim 0.93%.

http://www.corp.google.com/~yaowu/no_crawl/mulcontext.html

Change-Id: I15bc9697f6ff7829042911fe0c62930585d7e65d