our specified section plane; we then split them according to the
start and endpoints of each STrimBy, using de Castejau's algorithm.
These sections get projected (possibly in perspective, which I do
correctly) into 2d and exported.
Except, for now they just get pwl'd in the export files. That's the
fallback, since it works for any file format. But that's the place
to add special cases for circles etc., or to export them exactly.
DXF supports the latter, but very painfully since I would need to
write a later-versioned file, which requires thousands of lines of
baggage. I'll probably stick with arcs.
[git-p4: depot-paths = "//depot/solvespace/": change = 1936]
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