Remove internal usage of ClutterGeometry in StageWindow

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