clutter-stage: Don't emit "after-paint" when picking [performance]
after-paint" signal should only be emitted on real stage repaints
and not fake stage repaints used in picking. The latter might occur
at a much higher and inconsistent rate and is not what apps/toolkits
_NET_WM_FRAME_DRAWN care about.
Without this change, apps will receive an extra
_NET_WM_FRAME_DRAWN event for every call to
_clutter_stage_do_pick_on_view. So GTK apps for example would get
GtkTickCallback faster than the display's refresh rate.