Add debug annotations for the layout process

If we need to check that the layout sequence is correct in
terms of order of execution and with respect to caching, then
having a CLUTTER_DEBUG_LAYOUT debug flag would make things
easier.

clutter/clutter-debug.h

@@ -21,7 +21,8 @@ typedef enum {
   CLUTTER_DEBUG_SCRIPT          = 1 << 11,
   CLUTTER_DEBUG_SHADER          = 1 << 12,
   CLUTTER_DEBUG_MULTISTAGE      = 1 << 13,
-  CLUTTER_DEBUG_ANIMATION       = 1 << 14
+  CLUTTER_DEBUG_ANIMATION       = 1 << 14,
+  CLUTTER_DEBUG_LAYOUT          = 1 << 15
 } ClutterDebugFlag;
 
 #ifdef CLUTTER_ENABLE_DEBUG