wayland: Load a buffer from well known location for the cursor

Semantic changes to Wayland means that we cannot rely on the compositor
setting a pointer buffer for us if set it to nil. The first part of fixing
this is to create an shm buffer containing the bytes for our cursor.

The best way to do this currently is to load the cursor from the well known
location where weston instals its cursor images. The code to implemente this
was derivedlifted from the Wayland backend in GTK+.

clutter/wayland/clutter-backend-wayland.h

@@ -56,6 +56,9 @@ struct _ClutterBackendWayland
   struct wl_compositor *wayland_compositor;
   struct wl_shell *wayland_shell;
   struct wl_shm *wayland_shm;
+  struct wl_buffer *cursor_buffer;
+  gint cursor_x, cursor_y;
+
   GSource *wayland_source;
 
   /* event timer */