Extension of segmentation to 8 segments.

Also some further simplification following removal
of top node code.

There is an issue in regards to the shared file vp8cx.h
in regard to the roi_map as this interface assumes that
there are only 4 segments. I have left the value here as
4 for now meaning that the roi_map interface is broken
for VP9.

Note that this change would have been easier if I hadn't
had to search for hard wire instances of the number 4
and <= 3.

Change-Id: Ia8b6deea4be4dbd20deb1656e689dd43a5f190e8