wayland: don't leave root window values uninitialised
wayland: don't leave root window values uninitialised

Under Wayland we don't know the absolute position of the device but there are
some API calls that expect to get an root window position. Previously we were
not assigning any value to these out parameters potentially leaving the values
undefined.

This change returns the current surface relative position of the device.

Lyude/gtk-,ahodesuka/gtk,davidgumberg/gtk,ebassi/gtk,Adamovskiy/gtk,ahodesuka/gtk,alexlarsson/gtk,jessevdk/gtk,chergert/gtk,alexlarsson/gtk,Lyude/gtk-,chergert/gtk,grubersjoe/adwaita,Sidnioulz/SandboxGtk,Adamovskiy/gtk,ebassi/gtk,alexlarsson/gtk,jadahl/gtk,jigpu/gtk,jigpu/gtk,alexlarsson/gtk,davidgumberg/gtk,jadahl/gtk,bratsche/gtk-,Adamovskiy/gtk,alexlarsson/gtk,alexlarsson/gtk,jigpu/gtk,jessevdk/gtk,chergert/gtk,Sidnioulz/SandboxGtk,jadahl/gtk,jadahl/gtk,jigpu/gtk,jadahl/gtk,jessevdk/gtk,ahodesuka/gtk,grubersjoe/adwaita,jadahl/gtk,jessevdk/gtk,davidgumberg/gtk,ebassi/gtk,chergert/gtk,Lyude/gtk-,Distrotech/gtk2,msteinert/gtk,alexlarsson/gtk,Distrotech/gtk2,chergert/gtk,Distrotech/gtk2,davidgumberg/gtk,msteinert/gtk,ebassi/gtk,msteinert/gtk,jigpu/gtk,jadahl/gtk,chergert/gtk,Sidnioulz/SandboxGtk,Lyude/gtk-,Sidnioulz/SandboxGtk,jessevdk/gtk,grubersjoe/adwaita,Sidnioulz/SandboxGtk,ahodesuka/gtk,msteinert/gtk,Lyude/gtk-,Adamovskiy/gtk,Sidnioulz/SandboxGtk,ebassi/gtk,Adamovskiy/gtk,davidgumberg/gtk,Lyude/gtk-,ebassi/gtk,jigpu/gtk,jadahl/gtk,davidgumberg/gtk,chergert/gtk,bratsche/gtk-,Distrotech/gtk2,Adamovskiy/gtk,grubersjoe/adwaita,jessevdk/gtk,jigpu/gtk,ahodesuka/gtk,Lyude/gtk-,chergert/gtk,Adamovskiy/gtk,grubersjoe/adwaita,jessevdk/gtk,msteinert/gtk,grubersjoe/adwaita,grubersjoe/adwaita,bratsche/gtk-,Lyude/gtk-,alexlarsson/gtk,bratsche/gtk-,jigpu/gtk,bratsche/gtk-,grubersjoe/adwaita,davidgumberg/gtk,Adamovskiy/gtk,msteinert/gtk,Distrotech/gtk2,davidgumberg/gtk,ahodesuka/gtk,ahodesuka/gtk,ahodesuka/gtk,Distrotech/gtk2,bratsche/gtk-