Constructs a regular polygon with the given number of vertices, all
located on a circle of the specified radius around "center". The radius
is the actual distance from "center" to each vertex.
S2LoopmakeRegularLoop(S2Point center, S1Angle radius, int num_vertices)
Constructs a regular polygon with the given number of vertices, all located on a circle of the specified radius around "center". The radius is the actual distance from "center" to each vertex.