the same plumbing as the 2d vector output.
Also fix piecewise linear tolerance when the export scale factor is
not equal to one; have to scale the chord tol along with that.
[git-p4: depot-paths = "//depot/solvespace/": change = 2053]
formats, with the proper color and width. This may need a bit of
cleanup for stuff like the hidden line removal, which currently
loses the style.
Also fix a bug in the test for arcs of a circle. A second-order
Bezier with collinear control points really is an arc, but it's an
arc with infinite radius so stuff tends to blow up. So return false
for that one.
[git-p4: depot-paths = "//depot/solvespace/": change = 2030]
Jonathan Westhues