This incorporates two enhancements:
1. A simpler formulation for the area of a spherical triangle with one
vertex at the south pole.
2. Instead of calling atan2 for every point, we use the identity:
∑ arg(z) = arg(∏ z),
where z is a complex number, and only call atan2 once for each
polygon, instead opting for a cheaper complex number multiplication
for each point.
This is around 3x faster than the old implementation as measured by the
included benchmark.
Mike Bostock
Jason Davies