gtkcssshadowvalue: Use cairo_surface_create_similar_image
gtkcssshadowvalue: Use cairo_surface_create_similar_image

Calling this on an Xlib surface allows us to keep this in SHM memory,
giving it a potential speedup so we don't have to copy it to SHM memory
for the Composite when we mask later.

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