Major parts have been implemented as follows:
(1) Added BRF_UPDATE, LASTNRF_UPDATE, and NRF_UPDATE in firstpass.c;
(2) Added the handling for the scenario of
"cpi->common.show_existing_frame == 1" at the encoder;
(3) Added a new reference frame of BWDREF_FRAME;
(4) Have bwd-ref work with upsampled references.

Note that when the experiment of "ext_refs" turned on, this experiment
will be turned off automatically currently.

RD performance in Overall PSNR has been improved, compared against the
VP10 baseline:

lowres: Avg -3.312; BDRate -3.154
derflr: Avg -1.927; BDRate -1.176
midres: Avg -2.149; BDRate -2.001
hdres : Avg -0.567; BDRate -0.588

Change-Id: I4c06ff51cc20194bffbd4d2346e57ba3dcf6b62c