surface-actor: Restore drop shadow with server-side decorations
Commit 510cbef1 changed the logic in handle_update() for X11 window
actors to return early if the surface is not an X11 surface.
That works fine for plain Xorg, but on Xwayland, the surface is actually a Wayland surface, therefore function returns before actually updating the drop shadows of server-side decorations for X11 windows.
Change the test logic to restore drops shadows with Xwayland windows.