Merge: Minor fix to atan2 and triangulate

Fix reversed axes in `atan2`, but since it is actually just a change in the name of the variables (and doc) it should not break the clients.

Also sorting points of a polygon in `triangulate` completely invalidated some complex polygons.

Pull-Request: #1958
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Romain Chanoir <romain.chanoir@viacesi.fr>