xwayland: Call RRTellChanged if the RandR configuration may have changed

This makes sure RandR events are sent to interested clients as needed.
This was happening implicitly in some cases, but not in others, e.g. if
the root window size didn't change.

If this were to call RRTellChanged more often than necessary in some
cases, that should be harmless, as it only sends events if something
has actually changed since last time.

Should fix https://bugzilla.redhat.com/show_bug.cgi?id=1979892 .

v2:
* Call RRTellChanged at the very end of update_screen_size, just in
  case.

Reviewed-by: Olivier Fourdan <ofourdan@redhat.com>
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