GdkEvent: Fix introspection annotations on gdk_event_get(_root)_coords
GdkEvent: Fix introspection annotations on gdk_event_get(_root)_coords

These functions respect NULLs being passed, so the return values are
(nullable).

Adamovskiy/gtk,grubersjoe/adwaita,davidgumberg/gtk,chergert/gtk,jadahl/gtk,jigpu/gtk,alexlarsson/gtk,davidgumberg/gtk,alexlarsson/gtk,jigpu/gtk,davidgumberg/gtk,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,davidgumberg/gtk,alexlarsson/gtk,alexlarsson/gtk,jigpu/gtk,Adamovskiy/gtk,jigpu/gtk,Adamovskiy/gtk,jigpu/gtk,davidgumberg/gtk,davidgumberg/gtk,chergert/gtk,jadahl/gtk,chergert/gtk,davidgumberg/gtk,jadahl/gtk,jadahl/gtk,chergert/gtk,chergert/gtk,chergert/gtk,Adamovskiy/gtk,jigpu/gtk,grubersjoe/adwaita,grubersjoe/adwaita,jadahl/gtk,grubersjoe/adwaita,alexlarsson/gtk,Adamovskiy/gtk,alexlarsson/gtk,alexlarsson/gtk,jigpu/gtk,alexlarsson/gtk,chergert/gtk,davidgumberg/gtk,jigpu/gtk,Adamovskiy/gtk,chergert/gtk,jadahl/gtk,grubersjoe/adwaita,Adamovskiy/gtk,Adamovskiy/gtk,jadahl/gtk,jadahl/gtk