New previous coef context experiment

Adds an experiment to derive the previous context of a coefficient
not just from the previous coefficient in the scan order but from a
combination of several neighboring coefficients previously encountered
in scan order.  A precomputed table of neighbors for each location
for each scan type and block size is used. Currently 5 neighbors are
used.

Results are about 0.2% positive using a strategy where the max coef
magnitude from the 5 neigbors is used to derive the context.

Change-Id: Ie708b54d8e1898af742846ce2d1e2b0d89fd4ad5