Support a constant quality mode in VP9

Adds a new end-usage option for constant quality encoding in vpx. This
first version implemented for VP9, encodes all regular inter frames
using the quality specified in the --cq-level= option, while encoding
all key frames and golden/altref frames at a quality better than that.

The current performance on derfraw300 is +0.910% up from bitrate control,
but achieved without multiple recode loops per frame.

The decision for qp for each altref/golden/key frame will be improved
in subsequent patches based on better use of stats from the first pass.
Further, the qp for regular inter frames may also be varied around the
provided cq-level.

Change-Id: I6c4a2a68563679d60e0616ebcb11698578615fb3