[cogl] Add a PANGO debug flag for CoglPango use

The CoglPango code falls under the COGL "jurisdiction"; this means
that it cannot include Clutter headers unless strictly necessary.

The CoglPangoRenderer code was using the CLUTTER_NOTE() macro. Now
that COGL has it's own COGL_NOTE() similar macro, CoglPango should
use that and avoid including clutter-debug.h (which pulls in
clutter-private.h which in turn pulls in clutter-actor.h).

A new flag, COGL_DEBUG_PANGO, has been added to the COGL debug
flags.

clutter/cogl/common/cogl-debug.c

@@ -13,7 +13,8 @@ static const GDebugKey cogl_debug_keys[] = {
   { "material", COGL_DEBUG_MATERIAL },
   { "shader", COGL_DEBUG_SHADER },
   { "offscreen", COGL_DEBUG_OFFSCREEN },
-  { "draw", COGL_DEBUG_DRAW }
+  { "draw", COGL_DEBUG_DRAW },
+  { "pango", COGL_DEBUG_PANGO }
 };
 
 static const gint n_cogl_debug_keys = G_N_ELEMENTS (cogl_debug_keys);