Z layer code:
https://wiki.factorio.com/Types/RenderLayer
render_layer = "object",
is what buildings are, I think
floor
transport-belt
transport-belt-endings
corpse
floor-mechanics
has to be one of these. unfortunately, there seems to be no layer between belt and floor, so cables cannot be under belts without concrete covering it up, perhaps over belts with some added shadows or something
