Space view heuristics don't create 2D space view at paths where there's a 3D space view even if it is needed

[Wumpf]

Check cargo rerun ./examples/assets: Clearly there should be both a 2D & 3D space view at the root since the 3D space view. can't display the images.
However, the way space view heuristic is setup up right now, 2D & 3D "compete" for the same slot, and 3D happens to "win". Thus, not creating any 2D space view.

To be fixed as part of:

• Re-engineer the space-view heuristics engine #4388