GtkProgressBar: implement clipping
GtkProgressBar: implement clipping

This allows progress bars to have shadows.

https://bugzilla.gnome.org/show_bug.cgi?id=733361

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