doc: fix clutter_init() usage in examples

Make sure users get the idea that clutter_init()
has a return value that needs to be checked.

These were fixed via sed magic:

sed -i -s -e "s/clutter_init (.*)/\
if (& != CLUTTER_INIT_SUCCESS)\n    return 1/"\
 doc/*/*/*.{c,xml} doc/*/*.xml

http://bugzilla.clutter-project.org/show_bug.cgi?id=2574

doc/cookbook/examples/layouts-stacking-diff-sized-actors.c

@@ -14,7 +14,8 @@ main (int argc, char *argv[])
   ClutterColor *color;
   ClutterActor *rect;
 
-  clutter_init (&argc, &argv);
+  if (clutter_init (&argc, &argv) != CLUTTER_INIT_SUCCESS)
+    return 1;
 
   stage = clutter_stage_get_default ();
   clutter_actor_set_size (stage, 400, 400);