Fix EllipticalArc::pointAndDerivatives() · a11b5a27

This function was simply returning wrong values which weren't even
consistent with Taylor's theorem. Fix it, and write a test to check
Taylor's theorem holds.

* Add Ellipse::pointAndDerivatives() for completeness, and use it to
  implement EllipticalArc::pointAndDerivatives() internally.

* Reduce amount of redundant trig computations.

* Ensure pointAndDerivatives() doesn't return more derivatives than
  asked for, violating its documentation.

Fixes #83

Fix operator*/(IntPoint, double) · 667420bb

Define these operators explicitly and make them return the expected
Point. Otherwise the double implicitly converts to int and we get wrong
results.

Unfortunately they cannot be declared inline in the header without
restructuring the point.h and int-point.h headers.

Fixes #76

Fix build_from_sbasis() · 01da5841

Revert 7d345c62.

In particular note that the s^0 and s^1 terms of an SBasis describe a
cubic bezier, so the error between an SBasis and this cubic bezier is
the tail starting from the s^2 term, so the original value
of tail=2 was correct.

Add a regression test which checks that build_from_sbasis() actually
returns a curve that remotely approximates the given SBasis, which was
not the case before.

Fixes #51