Replace ClutterPoint by graphene_point_t

Remove the tests for ClutterPoint since it's
corresponding code moved to private ClutterStage
methods.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/458

src/backends/native/meta-backend-native.c

@@ -150,7 +150,7 @@ constrain_all_screen_monitors (ClutterInputDevice *device,
                                float              *x,
                                float              *y)
 {
-  ClutterPoint current;
+  graphene_point_t current;
   float cx, cy;
   GList *logical_monitors, *l;