and cubics. Also add that to the library interface.
It might have been better to use a single constraint for that,
plus all the line-curve or line-line cases, but it would break
backwards compatibility if I did that now, and perhaps be
confusing with the 'other' member (which is meaningless for
lines) anyways.
[git-p4: depot-paths = "//depot/solvespace/": change = 2141]
we're inconsistent (singular Jacobian). That's slow, so we should
provide a library interface to disable it.
[git-p4: depot-paths = "//depot/solvespace/": change = 1946]