gdkwindow: Simplify check for damaging offscreen windows
gdkwindow: Simplify check for damaging offscreen windows

If a window both has an impl and its impl_window is of type offscreen,
that must mean that it is the offscreen window, and the impl window is
itself. We can reduce the indirection here and make it more clear.

ahodesuka/gtk,Adamovskiy/gtk,jessevdk/gtk,Lyude/gtk-,jadahl/gtk,davidgumberg/gtk,jigpu/gtk,grubersjoe/adwaita,jigpu/gtk,grubersjoe/adwaita,jigpu/gtk,grubersjoe/adwaita,jessevdk/gtk,chergert/gtk,jigpu/gtk,jadahl/gtk,chergert/gtk,Lyude/gtk-,grubersjoe/adwaita,ahodesuka/gtk,ahodesuka/gtk,Adamovskiy/gtk,Adamovskiy/gtk,davidgumberg/gtk,alexlarsson/gtk,Lyude/gtk-,Adamovskiy/gtk,jessevdk/gtk,Lyude/gtk-,chergert/gtk,ahodesuka/gtk,Lyude/gtk-,chergert/gtk,Lyude/gtk-,jadahl/gtk,grubersjoe/adwaita,ahodesuka/gtk,davidgumberg/gtk,davidgumberg/gtk,davidgumberg/gtk,grubersjoe/adwaita,jigpu/gtk,Lyude/gtk-,davidgumberg/gtk,alexlarsson/gtk,alexlarsson/gtk,jigpu/gtk,ahodesuka/gtk,davidgumberg/gtk,jessevdk/gtk,davidgumberg/gtk,Lyude/gtk-,grubersjoe/adwaita,chergert/gtk,chergert/gtk,jadahl/gtk,ahodesuka/gtk,alexlarsson/gtk,alexlarsson/gtk,jessevdk/gtk,jigpu/gtk,jessevdk/gtk,jadahl/gtk,ahodesuka/gtk,jessevdk/gtk,alexlarsson/gtk,Adamovskiy/gtk,alexlarsson/gtk,jigpu/gtk,jadahl/gtk,chergert/gtk,jadahl/gtk,jadahl/gtk,Adamovskiy/gtk,grubersjoe/adwaita,chergert/gtk,alexlarsson/gtk,Adamovskiy/gtk,Adamovskiy/gtk