wayland/pointer: Track lifetime of current surface

Clear the pointer->current when the surface is destroyed.

https://bugzilla.gnome.org/show_bug.cgi?id=783113

src/wayland/meta-wayland-pointer.h

@@ -87,6 +87,7 @@ struct _MetaWaylandPointer
 
   ClutterInputDevice *device;
   MetaWaylandSurface *current;
+  gulong current_surface_destroyed_handler_id;
 
   guint32 button_count;
 };