Clean up debug features

Introduce a GTK_DEBUG_CHECK() macro and use it to check for
GTK_DEBUG flags everywhere. Also guard all such places by

gtk/gtktextiter.c

@@ -369,7 +369,7 @@ is_segment_start (GtkTextRealIter *real)
 static void
 check_invariants (const GtkTextIter *iter)
 {
-  if (gtk_get_debug_flags () & GTK_DEBUG_TEXT)
+  if (GTK_DEBUG_CHECK (TEXT))
     _gtk_text_iter_check (iter);
 }
 #else