This commit merges those parts of the CONFIG_NEW_MVREF
that specifically relate to choosing a better set of candidate
MV references into the NEWBESTREFMV experiment.

CONFIG_NEW_MVREF will then be used for changes relating
to the explicit coding of a cost optimized MV reference in the
bitstream as part of MV coding.

Change-Id: Ied982c0ad72093eab29e38b8cd74d5c3d7458b10

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

Alternative strategy for finding a list of candidate motion
vectors to use as reference values in mv coding and as
nearest and near.

Sort by sad in vp8_find_best_ref_mvs() rather than just
pick the best. Allow 0,0 as a best ref option but not a
nearest or near unless there are no alternatives.

Encode/Decode verified on at least some clips.

Some commented out experimental and stats code still in place.

Gain over existing code averages about 1% on derf (alll metrics)
with improvement on all clips. Other test results pending.

The entropy coding of the mode (nearest/near etc) still
depends upon and requires the old "findnear" code so
this needs looking at and may provide room for further gains.

Change-Id: I871d7cba1d1c379c4bad9bcccce1fb19c46b8247