window: Reinstate logic for should_use_csd()
window: Reinstate logic for should_use_csd()

The old should_use_csd() function would return FALSE if the GTK_CSD
environment variable is unset; the change in commit c5e5ee67490e7e7
made it return TRUE if GTK_CSD is unset. This has a cascade effect
on the window size, which causes invalid rectangles to bubble down
to Pixman.

https://bugzilla.gnome.org/show_bug.cgi?id=751140

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