Fix the flatten ground tool around cliffs

[CCTiberius]

When using the flatten ground tool, if you get near a cliff the tool will bug out and create unwanted ramps and heightened tiles which have to be manually lowered or removed with other tools as the flatten ground tool will just keep making them.

[Crimsonum]

I believe this happens when you use any of the slope tools near cliffs (flatten, raise, and lower terrain tools).