[Lightning] Use a single z-buffer texture for shadow maps on spot lights

[pedropaulosuzuki]

Description

The current implementation uses an omnidirectional shadow map (6 faces) for point lights and spot lights. When we're using a spot light, it will only generate a shadow for the region inside the cone, which means any texels outside that region are wasted. If we bake the projection matrix with the correct angle, we will have the best quality possible for a given resolution.

Here is a video I made earlier on the difference in resolution density between a tetrahedron mapping and a single optimized z-buffer. Even though we use a cube mapping currently, the logic is still similar.

0001-0030.mp4