paint volumes: CLUTTER_PAINT=paint-volumes debug option

This adds a debug option to visualize the paint volumes of all actors.
When CLUTTER_PAINT=paint-volumes is exported in the environment before
running a Clutter application then all actors will have their bounding
volume drawn in green with a label corresponding to the actors type.

clutter/clutter-debug.h

@@ -35,7 +35,8 @@ typedef enum {
 typedef enum {
   CLUTTER_DEBUG_DISABLE_SWAP_EVENTS     = 1 << 0,
   CLUTTER_DEBUG_DISABLE_CLIPPED_REDRAWS = 1 << 1,
-  CLUTTER_DEBUG_REDRAWS                 = 1 << 2
+  CLUTTER_DEBUG_REDRAWS                 = 1 << 2,
+  CLUTTER_DEBUG_PAINT_VOLUMES           = 1 << 3
 } ClutterDrawDebugFlag;
 
 #ifdef CLUTTER_ENABLE_DEBUG