Add a (failing) test for slicing at the same height of a horizontal surface attached to a volume. In this case, the loop isn't completed. #1672

Conflicts:

	xs/t/01_trianglemesh.t
Bugfix: incomplete slicing when a horizontal surface was tangent to the slicing plane and it shared an edge with an adjacent volume. #1672

Conflicts:

	xs/src/TriangleMesh.cpp