WMO material defs inappropriately override one another

[fallenoak]

In each WMOGroup, when we load the group, we're manipulating some properties on the defs coming over from MOMT-- things like assigning indoor/outdoor flags, etc.

To avoid unexpected overrides of these properties, rather than directly manipulate the original MOMT material def object, we should clone the material def and then manipulate it.

[timkurvers]

👍