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

js/ui/windowManager.js

@@ -1286,6 +1286,8 @@ const WindowManager = new Lang.Class({
     _sizeChangedWindow: function(shellwm, actor) {
         if (!actor.__fullscreenInfo)
             return;
+        if (this._resizing.indexOf(actor) != -1)
+            return;
 
         let actorClone = actor.__fullscreenInfo.clone;
         let targetRect = actor.meta_window.get_frame_rect();