[mutter] MetaWindowActor: assert that we have plugin manager on simple effect

[Marco Trevisan <marcotrevi src gnome org>]

commit 91e3a0b3a942163ed369254b69fdc5c879d0e841
Author: Marco Trevisan (Treviño) <mail 3v1n0 net>
Date:   Fri Oct 20 02:52:04 2017 -0500

    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 |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/src/compositor/meta-window-actor.c b/src/compositor/meta-window-actor.c
index de506aa..2a89a7f 100644
--- a/src/compositor/meta-window-actor.c
+++ b/src/compositor/meta-window-actor.c
@@ -1076,6 +1076,8 @@ 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: