Portals - disable frustum culling gizmos with preview camera

[lawnjelly]

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.

Fixes #50999

Notes

• This could be done more efficiently by marking the gizmos as debug somehow in the VisualServer, and culling the gizmos from the main camera, and other items from the preview camera. However, this would make the code required more complex, and I'm not convinced it is necessary for this use.
• If we have reports of a slowdown using the preview_camera and large scenes with lots of gizmos we could make the effort to do the above approach. But it seems unlikely.

[akien-mga]

Thanks!