Actor Physics Fixes (0.23.1 Alpha)

I found and fixed  more bugs in the code responsible for Actor-terrain interactions.

The first of these was causing terrain to remain after it's block or fluid was removed.

The second was a tiny bug where slopes around blocks were not being updated as blocks were added or removed.

Last is a bug that was causing Actors to ignore terrain in vertically looping worlds, if the view was well outside the world's true coordinates.  Basically, if the view stayed inside the vertical bounds of the world, without moving to a higher or lower looped copy, everything worked fine.  If the view moved entirely to one of the looped copies, all Actors would start falling endlessly.