reftests: Improve workaround for GtkGrid bug
reftests: Improve workaround for GtkGrid bug

... so that it works with wide separators. Or rather: with separators
that don't request 1px size but any other number. Do that by making the
placeholder request the same size by indeed stuffing (hidden) separators
in it.

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