gdkwindow: don't use last_slave for the source device if it is NULL
gdkwindow: don't use last_slave for the source device if it is NULL

Some backends do not have slave devices, which means last_slave may be
NULL. Use the current device as the source device if last_slave is NULL
when synthesizing a crossing event.

https://bugzilla.gnome.org/show_bug.cgi?id=692411

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