Improvements:
* Occluder3D is now an abstract type inherited by: ArrayOccluder3D, QuadOccluder3D, BoxOccluder3D, SphereOccluder3D and PolygonOccluder3D. ArrayOccluder3D serves the same purpose as the old Occluder3D (triangle mesh occluder) while the rest are primitives that can be used to manually place simple occluders.
* Occluder baking can now apply simplification. The "bake_simplification_distance" property can be used to set a world-space distance as the desired maximum error, set to 0.1 by default.
* Occluders can now be generated on import. Using the "occ" and "occonly" keywords (similar to "col" and "colonly" for colliders) or by enabling on MeshInstance3Ds in the scene's import window.
Fixes:
* Fixed saving of occluder files after bake.
* Fixed a small error where occluders didn't correctly update in the rendering server.
Bonus content:
* Generalized "CollisionPolygon3DEditor" so it can also be used to edit Resources. Renamed it to "Polygon3DEditor" since it was already being used by other things, not just colliders.
* Fixed a small bug in "EditorPropertyArray" where a call to "remove" was left after the "remove_at" rename.
* Added a new macro SNAME() that constructs and caches a local stringname.
* Subsequent usages use the cached version.
* Since these use a global static variable, a second refcounter of static usages need to be kept for cleanup time.
* Replaced all theme usages by this new macro.
* Replace all signal emission usages by this new macro.
* Replace all call_deferred usages by this new macro.
This is part of ongoing work to optimize GUI and the editor.
Added an occlusion culling system with support for static occluder meshes.
It can be enabled via `Project Settings > Rendering > Occlusion Culling > Use Occlusion Culling`.
Occluders are defined via the new `Occluder3D` resource and instanced using the new
`OccluderInstance3D` node. The occluders can also be automatically baked from a
scene using the built-in editor plugin.
