alt-tab: Enforce icon size

For applications with no proper desktop file, the window icon is
used as application icon in the tab switcher, but it won't have
the correct icon size. The current approach is to add additional
padding to these icons - the size turns out inconsistent with
other icons, but the icon appears sharp. For the dash it has been
decided that unsharp icons are less evil than differing icon sizes,
so icons are scaled up to the "right" size - for consistency, do the
same in the alt-tab switcher.

https://bugzilla.gnome.org/show_bug.cgi?id=643300

js/ui/altTab.js

@@ -820,7 +820,7 @@ AppIcon.prototype = {
         this.actor = new St.BoxLayout({ style_class: 'alt-tab-app',
                                          vertical: true });
         this.icon = null;
-        this._iconBin = new St.Bin();
+        this._iconBin = new St.Bin({ x_fill: true, y_fill: true });
 
         this.actor.add(this._iconBin, { x_fill: false, y_fill: false } );
         this.label = new St.Label({ text: this.app.get_name() });