as.mesh3d for spatials

[mdsumner]

currently we have

as.mesh3d.sfc <-function(x, ...

  as.mesh3d(TRI0(x),

but, that was a mistake. It needs to stick to the silicate way, and allow the new mesh3d types ie.

points = for POINT
segments = M/LINESTRING, M/POLYGON

in rare cases there'll be SF triangles but that stuff's not fully fleshed out in silicate. Otherwise, you can push sf through SC0/TRI0/DEL0 for the obvious (re)interpretations.

Qs

• do we bother with as.mesh3d.sfc_POINT - ah, no SC0 already handles to just palm off to that
• update as.mesh3d.SC0 and it's mostly done
• make sure consistency (polygons are lines) throughout

[mdsumner]

that was pretty easy