Changes to maximum gf/arf interval.

Changes to maximum gf/arf interval.

This patch puts in an adjustment to the maximum gf/arf
interval based on the active q range.  It sets a fixed
baseline maximum of 16 but can drop this down to 12 at
lower q. This required some re-ordering in the first pass
code to insure we have a Q range estimate before defining
the first gf sequence.

The main gains seed are int he STD hd set on 50fps clips
where previously the interval could rise as high as 25.
On the std hd clip the gains are around 2.8% with limit set
to 300 frames.

When combined with the one shot rate control flags we get
combined of:

derf 1.55% (limit300), yt 7.25%, hd 5.17% std-hd 5.84% (limit300)

Change-Id: Ib380d51354511f2ff0f171a8df4e74291c0421f9