Improve ScreenToScene function for non-flat planes

[JShep1]

@chapulina noticed in the PR for the tape measure tool (#456) that the end points for the tape measure are placed in unusual places for a non-flat plane case (like rolling_shapes.sdf). The logic for placing the end points makes use of the ScreenToScene function in Scene3D.cc

I believe there's an underlying assumption that the plane is flat and at the origin(?), although I could be wrong about that. Below is an example that @chapulina recorded.