In the experiment of EXT_REFS, add the use of the 3 extra reference
frames, including the 2 forward reference frames LAST2 and LAST3, and
the one backward reference frame BWDREF, for the encoding.

For lowres, EXT_REFS achieves following bitrate saving on overall PSNR
compared against baseline:
Avg: -4.581  BDRate: -4.358

Change-Id: Ib15b65681c39892ddfce5e4107598c0a22317ccd