ui: Reset workspace thumbnail porthole on monitor changes

The porthole will not be destroyed when the scale factor changed.
That makes workspace thumbnail porthole still wrong size in the first
seeing after the scale factor changed.

https://bugzilla.gnome.org/show_bug.cgi?id=765011

js/ui/workspaceThumbnail.js

@@ -676,6 +676,12 @@ const ThumbnailsBox = new Lang.Class({
         this._settings = new Gio.Settings({ schema_id: OVERRIDE_SCHEMA });
         this._settings.connect('changed::dynamic-workspaces',
             Lang.bind(this, this._updateSwitcherVisibility));
+
+        Main.layoutManager.connect('monitors-changed', Lang.bind(this, function() {
+            this._destroyThumbnails();
+            if (Main.overview.visible)
+                this._createThumbnails();
+        }));
     },
 
     _updateSwitcherVisibility: function() {