window: Fix child wfh size request
window: Fix child wfh size request

We need to subtract border_width from the size we're passing to the
children hfw functions as those are added by ourselves.

Fixes the window-border-width.ui reftest.

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