Instability near corners in non-convex meshses

[maplant]

As shown in mgf_demo/balls, some objects near the edge of the "box" exhibit instability and sink far to deeply into the floor. I have a few theories on why this happens, more than likely it is due to considering the wall and floor as the same physical object.

[maplant]

As I wrote in the commit message, this was caused by representing two points of contact with a convex, discontinuous surface with only one constraint.