Simplification of MVref search.

As we are no longer able to sort the candidate
mvrefs in both encoder and decode and given
that the cost of explicit signalling has proved
prohibitive, it no longer makes sense to find more
than 2 candidates.

This patch:

Modifies and simplifies add_candidate_mv()

Removes the forced addition of a 0 vector in the
MAX_MV_REF_CANDIDATES-1 position (in preparation
to reducing MAX_MV_REF_CANDIDATES to 2).

Re-orders the addition of candidates slightly.

This actually gives small gains (circa 0.2% on std-hd)

A subsequent patch will remove NEW_MVREF experiment,
reduce MAX_MV_REF_CANDIDATES to 2 and remove distance
weights as these are implicit now in the order.

Change-Id: I3dbe1a6f8a1a18b3c108257069c22a1141a207a4