Function S2Polygon.project

If the given point is contained by the polygon, return it. Otherwise return the closest point on the polygon boundary. If the polygon is empty, return the input argument. Note that the result may or may not be contained by the polygon. "x" should be unit length.

s2.util.math.vector.Vector!(double,3L) project (
  in const(s2.util.math.vector.Vector!(double,3L)) x
);