The tool of ext-comp-refs adds the uni-directional compound reference
prediction. In details, 3 pairs of uni-direcitonal compound references
are added for the comp ref prediction:
(LAST_FRAME, LAST2_FRAME),
(LAST_FRAME, GOLDEN_FRAME), and
(BWDREF_FRAME, ALTREF_FRAME).

This new tool of ext-comp-refs will eventually overwrite
one-side-compound and have the two coding tools to merge to one.

It achieves -0.35 ~ -0.55% coding gains in BDRate, compared against
AV1 baseline with the default experiments on, but without
one-sided-compound. It achieves -0.2% ~ -0.3% coding gains when
one-sided-compound is on. It achieves larger gains on higher
resolution.

Change-Id: Icbdb16e97b96aaebaf2213f5f72d5331e2e358eb