vp9: add fate sample for parallelmode.

This disables backward probability updates, which makes the codec more
friendly for frame-level multi-threading.

Signed-off-by: Anton Khirnov <anton@khirnov.net>
vp9: add a new segmentation sample.

The old one didn't use segmentation. One uses segmentation in all frame
types (--aq-mode=1), and the other uses all segmentation features, but
only in inter frames (mbgraph).

Signed-off-by: Anton Khirnov <anton@khirnov.net>