testgtk: Respect display cursor size limitations
testgtk: Respect display cursor size limitations

Instead of hardcoding 64, ask the display about the
maximal supported cursor size.

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