Added 3 more reference frames for inter prediction.

Under the experiment of EXT_REFS: LAST2_FRAME, LAST3_FRAME, and
LAST4_FRAME.

Coding efficiency: derflr +1.601%; hevchr +1.895%
Speed: Encoder slowed down by ~75%

Change-Id: Ifeee5f049c2c1f7cb29bc897622ef88897082ecf
26 files changed