Blender Git Loki

Cycles X: add camera visibility and shadow catcher options for lights

* Camera visibility makes it so lights can be directly visible as objects
in the scene. This is disabled by default but can be enabled now. Be
aware that aliasing can be a problem with directly visible bright lights.
* The shadow catcher works similar to emissive triangles on meshes. It's
useful when you for example use light objecs for the headlights of a car,
where you want their reflection on the road to be composited in.

For performance, I saw no difference outside of timing noise.

Differential Revision: https://developer.blender.org/D12141