libvpx

Changes to use of rectangular partitions.

Changes to allow more use of rectangular partitions at
speeds 1 and 2 for content classed by the first pass as
animation and for blocks near the active image edge.

This has quite a big impact in quality for the animated
test sequence but also hurts encode speed for speed 2.

For other content types the impact on both speed and
quality is small.

Added some plumbing for detection of internal vertical
image edges.

Change-Id: I3fc48de2349f8cb87946caaf0b06dbb0ea261a9a