also means that closed contours will get output as a single path
now, vs. one open path per Bezier segment before.
I've simplified the 2d/3d wireframe export targets somewhat; they
now support only Beziers, without an additional special case for
line segments. The performance penalty for that should not be worth
caring about, since that's infrequent.
And fix a memory leak in FindOuterFacesFrom(), fix ugly output of
filled triangles in PDF (because the line join style did bad things
on long skinny triangles), fix non-zero Z coordinates for exported
views or sections in DXF or STEP.
[git-p4: depot-paths = "//depot/solvespace/": change = 2061]
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