libvpx

Alter Speed 3.

This patch deletes the variance based speed three partitioning.
Speed 3 now uses the same partitioning method as speed 2
but with some stricter conditions.

The speed and quality are now somewhere between speeds 2 and 4
whereas before it was worse in both than speed 4.

Change-Id: Ia142e7007299d79db3ceee6ca8670540db6f7a41