Extend surface by negative value not working on simple surface R7(SR13)

We don’t make fixes that could make things worse in the current stable version. But only @rajaa can tell us how big a change this was and whether it feels safe to put in Rhino 7. Also, Rhino 8 WIP is available to you now, so you can take advantage of this (and may other) improvements. See Welcome to Serengeti for details.