Deferred rendering currently doesn't support occlusion culling. This PR
implements it in a straightforward way, mirroring what we already do for
the non-deferred pipeline.
On the rend3 sci-fi base test scene, this resulted in roughly a 2×
speedup when applied on top of my other patches. For that scene, it was
useful to add another option, `--add-light`, which forces the addition
of a shadow-casting light, to the scene viewer, which I included in this
patch.
8de6b16e9d