Implement occlusion culling

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.

modules/raycast/config.py

@@ -0,0 +1,12 @@
+def can_build(env, platform):
+    if platform == "android":
+        return env["android_arch"] in ["arm64v8", "x86", "x86_64"]
+
+    if platform == "javascript":
+        return False  # No SIMD support yet
+
+    return True
+
+
+def configure(env):
+    pass