MetaWindowActor: assert that we have plugin manager on simple effect

It looks that there are some extensions that run a Mainloop on startup,
causing to dispatch a clutter paint before the compositor is even available.
In such scenario a MetaWindow could try to start a simple effect
using a compositor plugin which is not there yet.

Then in order to catch these bugs we can now assert that the expected
conditions are valid, so that gnome-shell will provide a dumpstack to
debug the real offending JS code.

https://bugzilla.gnome.org/show_bug.cgi?id=789223

src/compositor/meta-window-actor.c

@@ -1069,6 +1069,8 @@ start_simple_effect (MetaWindowActor  *self,
   gint *counter = NULL;
   gboolean use_freeze_thaw = FALSE;
 
+  g_assert (compositor->plugin_mgr != NULL);
+
   switch (event)
   {
   case META_PLUGIN_NONE: