draw only visible arc portions, support animation & other improvements

other improvements:
* use machine epsilon to support drawing curves when points are close to zero
* Point class can construct via std::complex<double> & std::polar()
* move Point class to cairoPoincare.h library
* drawPoint(), drawLine() and drawUnitCircle() can accept a 3rd-party context
* 3rd-party Cairo context tested with testAnimation.cpp (not integrated into build scripts)