libvpx

Further speed trade off adjustments.

Small speed gain for speed 1.

Quality is generally a little up for speed 2.

Speed 3 was similar to speed 4 but now positioned more
evenly between 2 and 4 speed and quality wise.
   (opsnr +5.6% ssim +8.25% across all sets)

Speed 4 is a little slower than before but sizable quality gains.
   (opsnr +3.7% ssim +6.8% across all sets)

The code has been cleaned up a bit so that for each incremental
speed step changes over the previous speed step are applied.
This makes it easier to see what is changing from one setting to
the next.

Change-Id: I2d98d0d6230af23486adaec01908f58942a7cdeb