wayland: wrong position for (top|bottom):(right|center)

The notification box position is wrongly calculated when extra panels taking desktop space are placed on the left or top and the notification is set to display on the top|bottom:right|center:

As a side note: have been testing xfce-git with wayfire-git and I'm amaze at how well it is working already, thanks to all the people involved! The decision to support wayfire was a great choice, this compositor gives xfce an extra modern edge.