* 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.
