empty (no trims) surfaces. It will generate a screwy bounding box,
which will make things break numerically later.
[git-p4: depot-paths = "//depot/solvespace/": change = 1979]
of revolution, and put them in the same form as if they had been
draw by an extrusion (so that we can use all the same special case
intersection curves).
And add code to merge coincident faces into one. That turns out to
be more than a cosmetic/efficiency thing, since edge splitting
fails at the join between two coincident faces.
[git-p4: depot-paths = "//depot/solvespace/": change = 1965]
