libvpx

Change to arf boost calculation.

In this commit I have added an experimental function
that tests prediction quality either side of a central position
to calculate a suggested boost number for an ARF frame.

The function is passed an offset from the current position and
a number of frames to search forwards and backwards.
It returns a forward, backward and compound boost number.

The new code can be deactivated using #define NEW_BOOST 0

In its current default state the code searches forwards and backwards
from the proposed  position of the next alt ref.

The the old code used a boost number calculated by scanning forward
from the previous GF up to the proposed alt ref frame position.

I have also added some code to try and prevent placement of a gf/arf
where there is a brief flash.

Change-Id: I98af789a5181148659f10dd5dd2ff2d4250cd51c