Using a non-zero slope in all tests causes some normal calculations
at the end, which in turn exposed a number of places where the trimesh
invariants were not properly maintained. This fixes all the problems
exposed by the current tests (there were several, mostly related
to how to reassign the representative edge for a vertex when
triangles and edges are collapsed.
When insetting a square face with a slope, not all center
vertices got raised. This fixes that.
Also disabled the grid inset test, which doesn't work yet.
Based on python code from Henrik Dick, this libray function
calculates a Straight Skeleton, and hence, deals properly
with cases where the advancing inset geometry would overlap
or pass through opposite edges. It still needs work but the
basic tests pass.
This function will be used for edge and face bevels but is
not yet hooked up for that.
