window: Use CAIRO_OPERATOR_SOURCE to clear to background
window: Use CAIRO_OPERATOR_SOURCE to clear to background

Some windows, like GtkWindow and some other apps, set a transparent
background. The guarantee for begin_paint_region is that there should
be a full clear to the specified background color, not a composite
against what was there before.

This fixes repaint artifacts in Wayland and Weston in a better way than
76922c169f0699d9b85ef3eff2170f2cede1782c.

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