lets us export open curves, if the user drew them that way.
Also increase the limits on how many pwls we will generate for a
single curve.
[git-p4: depot-paths = "//depot/solvespace/": change = 1854]
some magic numbers. This would be trivial to break, but still more
difficult than patching the binary to skip the check...
[git-p4: depot-paths = "//depot/solvespace/": change = 1853]
where it was, move the export (DXF, PNG, STL) stuff to export.cpp,
and move the regen/solve stuff to generate.cpp.
[git-p4: depot-paths = "//depot/solvespace/": change = 1825]
Jonathan Westhues