stage: ensure correct stage is current when picking
stage: ensure correct stage is current when picking

_clutter_stage_do_pick called by interactive picking and
clutter_stage_get_actor_at_pos could be accidentally reading back the wrong
actor id's if an other stage has had a more recent render due to animation.

This should resolve some multi stage / ClutterGtk related pick id warnings.

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