When accessing properties on ClutterActor for size and position there is
a notable access time overhead. This overhead adds considerable user lag
when opening the overview if many windows are open.
This is primarily due to these properties being accessed while sorting
WindowClone instances by their window's center for placement in the
overview. By pre-computing this center value only once when
initializing WindowClone, the induced lag can be significantly reduced.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/763
(cherry picked from commit d91927674d4ea5ae7c7e77ace28f5939553147d5)
072a9a4842