Xwayland views are aware of their coordinates, so validating transaction
completions should take into account the reported coordinates of the
view. Prior to this commit they didn't, and matching dimensions would
suffice to validate the transaction.
Also introduced `transaction_notify_view_ready_immediately` to support
the fix from d0f7e0f without jumping through hoops to figure out the
geometry of an `xdg_shell` view.