libvpx

Enable dual arf with constant q.

Add second level arf Q adjustment when using dual arfs
in constant Q mode.

Previously in constant Q mode enabling dual arf hurt by ~5%
but with this change the average benefit is ~1-1.5% with some
mid range data points up ~10%.

Note however that it still hurts on some clips including
some very low motion show content.

Change-Id: I5b7789a2f42a6127d9e801cc010c20a7113bdd9b