A cluster of torches should not flicker uniformly

[depressed-pho]

It should really be computed separately for each light sources (i.e. torches) but sadly that isn't possible. As a plan B we can generate a three dimensional Perlin noise and use its depth directly as the intensity boost. It may be quite expensive to compute but fortunately the spacial frequency should be very low in this case, which means we can probably do it in the vertex shader without getting too much weird artifacts.