Change to MV reference search.

This patch reduces the cpu cost of the MV ref
search by only allowing insert for candidates
that would be in the current top 4.

This could alter the outcome and slightly favors
near candidates which are tested first but also
limits the worst case loop count to 4 and means in
many cases it will drop out and not happen.

Change-Id: Idd795a825f9fd681f30f4fcd550c34c38939e113