Ray3d::intersect_plane is not well document w.r.t. whether it reports backface hits as hits or not. i had to read the source and it does, and theres no option to have it Not do that, so im having to duplicate the method. would be nice to have an option for this and better docs

Reported by @atlv24.

I agree: we should make this configurable (probably just a bool argument), and clearly document the behavior.