Only handle viewable windows in collect_native_child_region
Only handle viewable windows in collect_native_child_region

There is no need to handle non-viewable windows and additionally
they may not have the clip region set yet.

davidgumberg/gtk,jigpu/gtk,jessevdk/gtk,msteinert/gtk,nacho/gtk-,chergert/gtk,simokivimaki/gtk,ahodesuka/gtk,Distrotech/gtk2,chipx86/gtk,Adamovskiy/gtk,Distrotech/gtk2,Adamovskiy/gtk,jessevdk/gtk,msteinert/gtk,simokivimaki/gtk,bratsche/gtk-,Sidnioulz/SandboxGtk,bratsche/gtk-,jadahl/gtk,msteinert/gtk,jadahl/gtk,Distrotech/gtk,alexlarsson/gtk,davidt/gtk,jigpu/gtk,jigpu/gtk,davidgumberg/gtk,Unity-Technologies/gtk,chergert/gtk,chergert/gtk,jigpu/gtk,alexlarsson/gtk,jessevdk/gtk,davidt/gtk,Distrotech/gtk2,jadahl/gtk,davidgumberg/gtk,ahodesuka/gtk,davidt/gtk,bratsche/gtk-,Lyude/gtk-,Distrotech/gtk,Sidnioulz/SandboxGtk,Lyude/gtk-,grubersjoe/adwaita,davidt/gtk,Sidnioulz/SandboxGtk,simokivimaki/gtk,ebassi/gtk,ahodesuka/gtk,grubersjoe/adwaita,alexlarsson/gtk,alexlarsson/gtk,msteinert/gtk,alexlarsson/gtk,johne53/MB3Gtk-2,grubersjoe/adwaita,johne53/MB3Gtk-2,davidgumberg/gtk,ahodesuka/gtk,Lyude/gtk-,Lyude/gtk-,alexlarsson/gtk,Distrotech/gtk,nacho/gtk-,zsx/gtk,grubersjoe/adwaita,chergert/gtk,jessevdk/gtk,ahodesuka/gtk,simokivimaki/gtk,davidt/gtk,jigpu/gtk,zsx/gtk,grubersjoe/adwaita,Lyude/gtk-,davidt/gtk,johne53/MB3Gtk-2,Adamovskiy/gtk,msteinert/gtk,Lyude/gtk-,Sidnioulz/SandboxGtk,chergert/gtk,davidgumberg/gtk,chergert/gtk,ahodesuka/gtk,grubersjoe/adwaita,jadahl/gtk,simokivimaki/gtk,msteinert/gtk,nacho/gtk-,ebassi/gtk,ebassi/gtk,alexlarsson/gtk,ebassi/gtk,jadahl/gtk,Unity-Technologies/gtk,Unity-Technologies/gtk,Unity-Technologies/gtk,Adamovskiy/gtk,chipx86/gtk,bratsche/gtk-,jessevdk/gtk,bratsche/gtk-,chipx86/gtk,simokivimaki/gtk,alexlarsson/gtk,Distrotech/gtk2,ebassi/gtk,davidgumberg/gtk,jigpu/gtk,Distrotech/gtk,nacho/gtk-,jigpu/gtk,jadahl/gtk,zsx/gtk,bratsche/gtk-,Adamovskiy/gtk,Sidnioulz/SandboxGtk,Distrotech/gtk2,jadahl/gtk,zsx/gtk,chergert/gtk,chergert/gtk,jessevdk/gtk,johne53/MB3Gtk-2,davidgumberg/gtk,ebassi/gtk,jessevdk/gtk,Adamovskiy/gtk,Sidnioulz/SandboxGtk,Lyude/gtk-,Adamovskiy/gtk,Distrotech/gtk,Unity-Technologies/gtk,Distrotech/gtk2,ahodesuka/gtk,grubersjoe/adwaita,chipx86/gtk,davidgumberg/gtk,jigpu/gtk,chipx86/gtk,Lyude/gtk-,Adamovskiy/gtk,zsx/gtk,grubersjoe/adwaita,johne53/MB3Gtk-2,jadahl/gtk,nacho/gtk-,ahodesuka/gtk