wayland/pointer-constraints: Adjustments for subsurface support

After the commit "wayland/subsurface: Implement
meta_wayland_surface_get_window()" subsurfaces are supported. Adjust
some comments and fix a warning that could occur when closing a window.
15 jobs for !2717 with issue2223-2 in 22 minutes and 5 seconds (queued for 1 second)
merge request