gdkwindow-win32.c: Fix build on Visual C++
gdkwindow-win32.c: Fix build on Visual C++

Since MSVC is more strict about the types that we try to do a compare, cast
old_ptr as HWND, as we know that it is a pointer...

https://bugzilla.gnome.org/show_bug.cgi?id=733768

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