Use above/left (instead of previous in scan-order) as token context.
Pearson correlation for above or left is significantly higher than for previous-in-scan-order (absolute values depend on position in scan, but in general, we gain about 0.1-0.2 by using either above or left; using both basically just makes this even better). For eob branch skipping, we continue to use the previous token in scan order. This helps about 0.9% on derf after re-training on a limited data set. Full re-training and results on larger-resolution clips are pending. Note that this commit breaks trellis, so we can probably get further gains out of it by fixing trellis at some later point. Change-Id: Iead68e296fc3a105cca746b5e3da9555d6010cfe