by ensuring `KernelLight.lightgroup` is properly assigned in
`device_update_light()`. The value is later retrieved via
`lamp_lightgroup(kg, lamp)`.
`light_manager->device_update()` is called after
`background->device_update()`, so the background light group should
already been assigned when `device_update_lights()` is called.
Pull Request: https://projects.blender.org/blender/blender/pulls/120714
8e5840359f