Vulkan: OmniLight3D/SpotLight3D projectors not reimplemented yet

[Calinou]

Godot version: master 80a4d0f

OS/device including version: Fedora 33, GeForce GTX 1080 (NVIDIA 460.32.03)

Issue description:

#45023 removed light projector support to improve shader occupancy, letting the GPU make better use of parallelism.

reduz said that light projectors should be reimplemented, but placed behind an opt-in project setting. The reason for doing this is that projector support increases the forward renderer's base cost, and therefore will slow down rendering even if no projectors are used at all.

Steps to reproduce: Look for light projectors in the master branch 🙂

Minimal reproduction project: N/A