gdk: Make background changes queue a repaint
gdk: Make background changes queue a repaint

For client-side windows, we need to queue a repaint when the background
changes. For native windows, the windowing system does take care of it,
but client-side windows are our own, so we gotta do it manually.

https://bugzilla.gnome.org/show_bug.cgi?id=652102

Unity-Technologies/gtk,johne53/MB3Gtk-2,johne53/MB3Gtk-2,Unity-Technologies/gtk,johne53/MB3Gtk-2,Distrotech/gtk,Unity-Technologies/gtk,Distrotech/gtk,Unity-Technologies/gtk,Distrotech/gtk,Distrotech/gtk,johne53/MB3Gtk-2,Unity-Technologies/gtk,johne53/MB3Gtk-2,Distrotech/gtk