gdk: master clock: only process mapped & realized stages
gdk: master clock: only process mapped & realized stages

When using Clutter embed inside a Gtk application, a stage might end
up realized but not visible. In this case we might discard doing any
kind of animation processing.

https://bugzilla.gnome.org/show_bug.cgi?id=754671

jigpu/clutter,jigpu/clutter,Distrotech/clutter,GNOME/clutter,GNOME/clutter,Distrotech/clutter,GNOME/clutter,jigpu/clutter,Distrotech/clutter,GNOME/clutter,jigpu/clutter,Distrotech/clutter,jigpu/clutter,Distrotech/clutter,GNOME/clutter,jigpu/clutter,Distrotech/clutter