How it works
The globe and the flat map are one surface. It is cut along the seam (red) and opened like
a cylinder, so Unwrap shows where every part of the map comes from. The map's poles (amber) are
points on the globe and whole lines on the map.
The same vertices are drawn two ways. Vertex by vertex projects each vertex and joins them
with straight lines on the map, as planar GIS code does. On the sphere joins them with great
circles, cuts at the seam and closes round the poles, as
d3-geo and s2 do. There, the winding order decides
which side of a ring is inside: d3 wants clockwise, GeoJSON and s2 counterclockwise.
The map centre moves the seam and the poles; the data never moves.