libvpx

Adds a speed feature for fast 1-loop forw updates

Incorporates a speed feature for fast forward updates of
coefficients. This feature takes 3 values:
0 - use standard 2-loop version
1 - use a 1-loop version
2 - use a 1-loop version with reduced updates

Results: derfraw300 +0.007% (on speed 0) at feature value = 1
                    -0.160% (on speed 0) at feature value = 2

There is substantial speed up at speeds 2 and above for low
resolution sequences where the entropy updates are a big part
of the overall computations.

Change-Id: Ie96fc50777088a5bd441288bca6111e43d03bcae