wayland: only update toplevel state values if we have a valid state

We have this annoying situation here during the initial window
configuration where sometimes we have to update certain values and other
times they need to be ignored. 45324ce682cc2668e760849441e337798277b769
made sure to parse the toplevel state before we checked the initial
compositor values which is correct. But it's not enough because mpv can
have its own initial state (e.g. --fullscreen) and by moving it here it
would get overwritten in the toplevel state check and thus ignored. So
to have our cake and eat it too, we need to only update the internal
state values if we know mpv is in a valid state in the first place. So
either we've set the initial geometry already or the compositor has
given us some specific initial state to work with. Fixes #17819.