Find something to do for “2d objects embedded in 3d space”

[plut]

• These objects naturally arise as the result of a 3x3 matrix applied to a plane object.
• They could also prove useful in some cases, e.g. for convex hulls.

=> instead of forbidding such constructions, maybe we should define an “embedded in 3d” type (maybe even make this the general type for 2d objects). CSG would be limited to objects in the same plane.