a11y: Not compute extents for actors without an stage
a11y: Not compute extents for actors without an stage

This solves a crash on GNOME Shell, as compute the extents
for some StWidgets could lead to call st_widget_get_theme_node,
and it is a fatal error to call this on a widget that it not
beed added to a stage.

djdeath/clutter-android,heysion/clutter-clone,ebassi/clutter,GNOME/clutter,djdeath/clutter,kerrickstaley/clutter-vala,jigpu/clutter,collects/clutter,jigpu/clutter,djdeath/clutter,djdeath/clutter,collects/clutter,jigpu/clutter,GNOME/clutter,collects/clutter,djdeath/clutter-android,djdeath/clutter-multithreaded,djdeath/clutter-android,jigpu/clutter,ebassi/clutter,ebassi/clutter,GNOME/clutter,heysion/clutter-clone,kerrickstaley/clutter-vala,djdeath/clutter-android,GNOME/clutter,Distrotech/clutter,djdeath/clutter,jigpu/clutter,djdeath/clutter-multithreaded,Distrotech/clutter,heysion/clutter-clone,ebassi/clutter,jigpu/clutter,djdeath/clutter-android,Distrotech/clutter,Distrotech/clutter,djdeath/clutter,heysion/clutter-clone,djdeath/clutter,djdeath/clutter-multithreaded,ebassi/clutter,djdeath/clutter-multithreaded,Distrotech/clutter,GNOME/clutter,Distrotech/clutter,heysion/clutter-clone,kerrickstaley/clutter-vala,kerrickstaley/clutter-vala,collects/clutter,collects/clutter