stack-tracker: Keep override redirect windows on top

Since commit 6b5cf2eb, we keep override redirect windows on a layer
above regular windows in the clutter actor scene graph. In the X
server, and thus for input purposes, these windows might end up being
stacked below regular windows though, e.g. because a new regular
window is mapped after an OR window.

Fix this disconnect by re-stacking OR windows on top when syncing the
window stack with the compositor.

https://bugzilla.gnome.org/show_bug.cgi?id=780485