This patch expands the set of prev contexts used for video coding
from 3 to 4.

There is a small improvement of the order of 0.08% for derf and
0.15% on the HD set. The tests were rerun after the various merges
last week. There are two columns in each test - the first are the
results with the mbskip change, and the second with expanded contexts
added on top of that.

Derf:
http://www.corp.google.com/~debargha/vp8_results/explibvpx_newentropy_expcontext.html

HD:
http://www.corp.google.com/~debargha/vp8_results/explibvpx_hd_newentropy_expcontext.html

Rebased.

Broke up 80 char lines.

Change-Id: I82d2e72d054e530cbf5ce9aa0e6d85c582965675