Includes a few fixes and clean-ups that adds the ability
to use alt-ref frames in one-pass mode.
Whether alt-refs are actually used or not is controlled by a
macro USE_ALTREF_FOR_ONE_PASS in vp9_firstpass.c.

This first cut seems to improve derf by 15+% in 1-pass mode.
But further experiments with parameters are underway.

Change-Id: I78254421435478003367c788c7930d2dc4ee2816