Record distortion for each single ref in rd. Rank according to their
distortions. Then in compound search, drop the combination of ref
frames of the largest and second largest distortions

This patch shows neutral performance on google test using lowres
with 20 frame.

Local tests show ~5% speed up over baseline.

Change-Id: I722fe66a0551f5f8a044c57c55caa74e46db7ee8