shells: Add libweston-desktop API to query position and add to shells

We're going to need this to properly send xwayland events later, so add
API to get the current x,y co-ordinates of a shell surface and add it to
the kiosk and desktop shells.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>

include/libweston-desktop/libweston-desktop.h

@@ -113,6 +113,9 @@ struct weston_desktop_api {
 	 */
 	void (*set_xwayland_position)(struct weston_desktop_surface *surface,
 				      int32_t x, int32_t y, void *user_data);
+	void (*get_position)(struct weston_desktop_surface *surface,
+			     int32_t *x, int32_t *y,
+			     void *user_data);
 };
 
 void