default plugin: use the right type for varargs

Coordinates in Clutter are double, so twice the size of an int, and
if not correctly casted memory corruption occurs.

https://bugzilla.gnome.org/show_bug.cgi?id=673809

src/compositor/plugins/default.c

@@ -479,8 +479,8 @@ minimize (MetaPlugin *plugin, MetaWindowActor *window_actor)
                                          MINIMIZE_TIMEOUT,
                                          "scale-x", 0.0,
                                          "scale-y", 0.0,
-                                         "x", icon_geometry.x,
-                                         "y", icon_geometry.y,
+                                         "x", (double)icon_geometry.x,
+                                         "y", (double)icon_geometry.y,
                                          NULL);
       apriv->tml_minimize = clutter_animation_get_timeline (animation);
       data->plugin = plugin;