Plus a small code clean up. The experiment of EXT_REFS, compared against
the baseline, using Overall PSNR, now obtains a gain on lowres as:
Avg: -5.818; BDRate: -5.653

Compared against the previous EXT_REFS results on lowres, a tiny gain is
obtained as:
Avg: -0.047, BDRate: -0.063

(1) 780952 Add encoder first pass support to bi-prediction in EXT_REFS
(2) f91498 Add pred prob handling for new references in EXT_REFS
(3) e91472 Add decoder support for bi-direct prediction in EXT_REFS
(4) 0dbac9 Add encoder support to new references in EXT_REFS
(5) ad70cc Remove hard-coded number for EXT_REFS
(6) 9c1e2f Add the use of new reference frames at encoder in EXT_REFS
(7) 6d4fde Add the experiment flag of EXT_REFS

Change-Id: I26f7ca45b9ede7579fdb9d0d6a1a91f4334599bd