Fix a problem with screen changes and csd

When a new screen is set on a window, we unrealize it, to
recreate all the resources. But we don't reset the client_decorated
flag, so realize() doesn't call create_decoration() - which makes
sense, since the decoration already exists. But the side-effect
of create_decoration() is to select the rgba visual, and visuals
are per-screen.

Fix this by looking for the rgba visual in set_screen(), and
replacing it with the rgba visual for the new screen, if necessary.