MV reference changes

Extend experiment to use both vectors from MBs
coded using compound prediction as candidates.

In final sort only consider best 4 candidates
for now but make sure 0,0 is always one of them.

Other minor changes to new MV reference code.
Pass in Mv list to vp8_find_best_ref_mvs().

Change-Id: Ib96220c33c6b80bd1d5e0fbe8b68121be7997095