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