videomixer: Maintain z-order when new pad are added
videomixer: Maintain z-order when new pad are added

https://bugzilla.gnome.org/show_bug.cgi?id=701109

Distrotech/gst-plugins-good,Distrotech/gst-plugins-good,Distrotech/gst-plugins-good,Distrotech/gst-plugins-good,Distrotech/gst-plugins-good