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