This patch removed the experiment of BIDIR_PRED and merged the feature
into the experiment of EXT_REFS:

(1) Each frame now has up to 6 reference frames, namely
    LAST_FRAME, LAST2_FRAME, LAST3_FRAME, GOLDEN_FRAME, (forward) and
    BWDREF_FRAME, ALTREF_FRAME (backward);
    LAST4_FRAME has been removed;
(2) First pass still keeps the 8 updates:
    KF_UPDATE, LF_UPDATE, GF_UPDATE, ARF_UPDATE, OVERLAY_UPDATE, and
    BRF_UPDATE, LAST_BIPRED_UPDATE, BI_PRED_UPDATE;
(3) show_existing_frame==1 is supported in the experiment of EXT_REFS;
(4) New encoding modes are added for both single-ref and compound cases,
    through the use of the 2 extra forward references (LAST2 & LAST3)
    and the 1 extra backward reference (BWDREF).

RD performance wise, using Overall PSNR: Avg/BDRate
        Bipred only      Prev EXT_REFS    Current EXT_REFS with bipred
lowres: -3.474/-3.324    -1.748/-1.586    -4.613/-4.387
derflr: -2.097/-1.353    -1.439/-1.215    -3.120/-2.252
midres: -2.129/-1.901    -1.345/-1.185    -2.898/-2.636

If in vp10/encoder/firstpass.h, change BFG_INTERVAL from 2 to 3, i.e. to
use 2 bi-predictive frames than 1, a further improvement may be
obtained:
                 Current EXT_REFS with bipred
        1 bi-predictive frame    2 bi-predictive frames
lowres: -4.613/-4.387            -4.675/-4.465
derflr: -3.120/-2.252            -3.333/-2.516
midres: -2.898/-2.636            -3.406/-3.095

Change-Id: Ib06fe9ea0a5cfd7418a1d79b978ee9d80bf191cb