scrolledwindow: Use a fully transparent background for the overlay window
scrolledwindow: Use a fully transparent background for the overlay window

The background will be already given and updated in the GtkScrollbar widget,
plus the background remained static on the window since it is realized.

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