Add a CLUTTER_PAINT debug option to enable continous redrawing

When testing the performance of an application, it's often useful to
force it to continuously redraw instead of going idle to help measure
the frame rate. This just adds a CLUTTER_PAINT=continuous-redraw which
causes the master clock to queue a redraw on all of the stages
just before it prepares its source.

Reviewed-by: Robert Bragg <robert@linux.intel.com>
Reviewed-by: Emmanuele Bassi <ebassi@linux.intel.com>

clutter/clutter-debug.h

@@ -41,7 +41,8 @@ typedef enum {
   CLUTTER_DEBUG_REDRAWS                 = 1 << 2,
   CLUTTER_DEBUG_PAINT_VOLUMES           = 1 << 3,
   CLUTTER_DEBUG_DISABLE_CULLING         = 1 << 4,
-  CLUTTER_DEBUG_DISABLE_OFFSCREEN_REDIRECT = 1 << 5
+  CLUTTER_DEBUG_DISABLE_OFFSCREEN_REDIRECT = 1 << 5,
+  CLUTTER_DEBUG_CONTINUOUS_REDRAW       = 1 << 6
 } ClutterDrawDebugFlag;
 
 #ifdef CLUTTER_ENABLE_DEBUG