libvpx

Experimental rate control change.

When the codec in VBR (or cq) mode hits its max q limits and is
struggling to hit a target bandwidth, the bit target per frame collapses.

In the first instance normal frames cap out at the maximum allowed
Q and then the ARF and GFs do the same. This latter behavior is not
generally desirable as GFs and ARFs are only effective from a quality
and data rate perspective if they have at lease some level of -Q delta
compared to the surrounding frames.

In this patch I define a separate max Q for GFs and ARFs that is
derived from but somewhat lower than that defined for normal frames.
In effect there is a minimum Q delta that will always be available for
GFs and ARFs regardless of the target rate and MAXQ setting.

This may of course mean that the absolute lowest rate obtainable for
a given clip is somewhat higher.

Change-Id: I268868b28401900d0cd87e51e609cd3b784ab54a