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/reference/clutter/clutter-animation-tutorial.xml

@@ -299,7 +299,8 @@ main (int argc, char *argv[])
   ClutterAlpha     *alpha;
   ClutterActor     *stage, *actor;
 
-  clutter_init (&argc, &argv);
+  if (clutter_init (&argc, &argv) != CLUTTER_INIT_SUCCESS)
+    return 1;
 
   stage = clutter_stage_get_default ();