Using a MetaWindowActor's shaped texture as the source for window
clones means that if there are further MetaSurfaceActor children
(e.g. a wayland client using sub-surfaces) they don't get cloned.
This obviously wasn't an issue until wayland clients introduced the
possibility of having multiple MetaSurfaceActors under a
MetaWindowActor but there's no fundamental reason we can't clone the
toplevel actor.
WorkspaceThumbnail.WindowClone is the one class that was already using
the MetaWindowActor instead of the texture although it seems to have
been an unintended change in commit
8b99617513da1713ff88ca4abf54e6125405d31c.
https://bugzilla.gnome.org/show_bug.cgi?id=756715
7746f1a5c4