PBR and cell data for metallic & roughness

[Timothee_Chabat (Timothee Chabat (Kitware))]

you can see answers here for what is supported and what is not.

@andreasbuykx

If each material (or material class, e.g. concrete, masonry, soil, rock, … ) would require its own actor this would increase the number of actors considerably.

I think that should still be fine performance wise, even though it is clearly not the cleanest way to tackle this issue.