windowManager: Avoid reentrancy on the ::size-changed handler

windowManager: Avoid reentrancy on the ::size-changed handler

We have no guarantees on the number of ::size-changed signals that we may
receive, so the _sizeChangedWindow handler may run multiple times, which
leads to multiple calls to meta_plugin_size_change_completed(). So double
check the actor is not already being animated in the _sizeChangedWindow
handler to avoid reentrancy.

https://bugzilla.gnome.org/show_bug.cgi?id=777784