xwayland: Fix rendering

Since we get the ClientMessage after the surface is created, there's
no good way to synchronize the two streams. In this case, what we
need to do is delay the surface commit until after we get the
ClientMessage. Ideally, we'd be using a better surface system overall
where committing the surface didn't depend on what type it is, but
oh well, this is a good short-term hack for now.