Refined cluster occlusion & tile culling with PTV

[Jarkko Lempiainen]

Clusters are currently occlusion culled early in the pipeline with pre-calculated cluster bounding spheres, which avoids vertex transforms of occluded clusters. However, this is quite conservative culling because the bounding sphere can be quite a poor match with the cluster geometry. This can be particularly a problem with procedural or deforming geometry where the bounds need to be quite conservative. For passing clusters we should add more refined culling using cluster post-transform vertex (PTV) positions and AABB of the vertex NDCs, for both refined occlusion and tile culling.