[sldev] Hide geometry in Sculpted-prims

[Feilen]

For all the misgivings about sculpted prims, the greatest number are 
about how all objects have to have perfect UV-mapping, making it nearly 
impossible in most cases to export existing objects into the format.

What I propose would fix this entirely: Simply make the renderer not 
render geometry connecting to the origin, aka 0,0,0 or black color on 
the sculpt map. This would solve the problem simply because by default 
exporters paint unused geometry black, which ruins some potential 
models, but would make it very easy to simply not draw triangles with a 
point at 0,0,0 and greatly improve sculpty submission quality.