Add framework for supporting geometrical occluders within rooms, and add support for sphere occluders.
Includes gizmos for editing.
They also work outside the portal system.
In some situations looking out from an internal room it was possible to look back into the portal into the internal room.
This PR fixes this by keeping a single item 'stack' record of the last external room, and preventing recursing into this room. This also makes tracing significantly more efficient out of internal rooms, as there is no need to trace the external room multiple times.
When using the preview camera feature it turns out as well as culling the game objects, this also culls the editor gizmos from the preview camera, which makes the editor hard to use in this mode.
To get around this problem we simply disable frustum culling for GLOBAL portal_mode objects when in preview camera mode. This could be a bit slower in an editor scene with lots of gizmos but is the simplest way of solving the problem.
