from an extrusion, with piecewise linear trim curves for everything
(that are shared, so that they appear only once for the two
surfaces that each trims). No Boolean operations on them, and the
triangulation is bad, because gl seems to merge collinear edges.
So before going further, I seem to need my own triangulation code.
I have not had great luck in the past, but I can't live without it
now.
[git-p4: depot-paths = "//depot/solvespace/": change = 1899]
Jonathan Westhues