Remove internal usage of ClutterGeometry in StageWindow

The ClutterGeometry type is a poor substitute of cairo_rectangle_int_t,
with unsigned integers for width and height to complicate matters.

Let's remove the internal usage of ClutterGeometry and switch to the
rectangle type from Cairo.

https://bugzilla.gnome.org/show_bug.cgi?id=656663

clutter/clutter-private.h

@@ -235,6 +235,10 @@ void  _clutter_util_fully_transform_vertices (const CoglMatrix    *modelview,
                                               ClutterVertex       *vertices_out,
                                               int                  n_vertices);
 
+void _clutter_util_rectangle_union (const cairo_rectangle_int_t *src1,
+                                    const cairo_rectangle_int_t *src2,
+                                    cairo_rectangle_int_t       *dest);
+
 typedef struct _ClutterPlane
 {
   CoglVector3 v0;