Fixes some issues found by UBSAN and other misc things:
* Fixed memory leak on exit.
* Properly align ray packet buffer to 64 bytes.
* Added some compiler flags from Embree's build system.
* Fixed ray masks.
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.
