This experiment is to add ALTREF2_FRAME to allow 2 altref backward
predictions. Each video frame will then have up to 7 reference frames
to choose from:

(1) 4 forward predictive references, namely
LAST_FRAME, LAST2_FRAME, LAST3_FRAME, and GOLDEN_FRAME; and
(2) 3 backward predictive references, namely
BWDREF_FRAME, ALTREF2_FRAME, and ALTREF_FRAME.

The tool of "altref2" is built on top of the "ext_refs" experiment.

Change-Id: Idbb0bb53b43c5c2c7baf4959331fc5a31c77a118