[3.x] Remove vi_visible flag from Spatial.

[lawnjelly]

Can be replaced by is_visible_in_tree().

Now we have faster pre-calculated visible_in_tree flag (#107324) there is no need for the VisualInstance specific flag vi_visible which can be replaced by the more encompassing flag.

Here the methods to generate and accecss vi_visible are removed, and use cases are changed to use is_visible_in_tree().

Notes

• I haven't attempted to change visible_in_tree propagation here, that would be for another PR.

[lawnjelly]

Have renamed the function and added a comment. I hadn't noticed that it actually refreshes it on ENTER_WORLD rather than ENTER_TREE, but that is historical and I'm assuming working fine (I'll double check, but I'll avoid changing unless necessary to avoid regressions).

UPDATE:
NOTIFICATION_ENTER_WORLD gets sent during NOTIFICATION_ENTER_TREE for Spatial so it should be fine. 👍

[Ivorforce]

Makes sense to me now. Thanks for the changes!

I'm not super convinced that _update_server_visibility_and_xform is only called if effectively_visible != was_effectively_visible — I have a feeling we might be sending NOTIFICATION_VISIBILITY_CHANGED notifications into it without checking effective visibility. I might be wrong.
But the function itself looks sound to me.

[lawnjelly]

Thanks!