Explicit MV reference experiment.

Coding and costing of mv reference signal.

Issues in updating MV ref with COMPANDED_MVREF_THRESH
to be resolved. Ideally the MV precision should be defined based
on absolute MV magnitude not as now the MV ref magnitude.

Update to mv counts moved into bitstream.c because otherwise
if the motion reference is changed at the last minute the encoder
and decoder get out of step in terms of the counts used to update
entropy probs.

Code working on a few test clips but no results yet re benefit vs
signaling cost and no tuning of red loop to test lower cost alternatives
based on the available reference values.

Patch 3. Added check to make sure we don't pick a reference
that would give rise to an uncodeable / out of range residual.

Patch 6-7: Attempt to rebase. OK to submit but best to leave flag off for now.

Patch 9. Remove print no longer needed.

Change-Id: I1938c2ffe41afe6d3cf6ccc0cb2c5d404809a712