wayland: Add a cursor cache

We avoid creating a new GdkCursor object every time
_gdk_wayland_display_get_cursor_for_name/type() is called.

gdk/wayland/gdkdisplay-wayland.h

@@ -65,6 +65,7 @@ struct _GdkWaylandDisplay
   struct wl_data_device_manager *data_device_manager;
 
   struct wl_cursor_theme *cursor_theme;
+  GSList *cursor_cache;
 
   GSource *event_source;