debug: Adds a COGL_DEBUG=wireframe option

This adds a COGL_DEBUG=wireframe option to visualize the underlying
geometry of the primitives being drawn via Cogl. This works for triangle
list, triangle fan, triangle strip and quad (internal only) primitives.
It also works for indexed vertex arrays.

cogl/cogl-debug.h

@@ -56,7 +56,8 @@ typedef enum {
   COGL_DEBUG_DISABLE_BLENDING = 1 << 23,
   COGL_DEBUG_TEXTURE_PIXMAP   = 1 << 24,
   COGL_DEBUG_BITMAP           = 1 << 25,
-  COGL_DEBUG_DISABLE_NPOT_TEXTURES = 1 << 26
+  COGL_DEBUG_DISABLE_NPOT_TEXTURES = 1 << 26,
+  COGL_DEBUG_WIREFRAME        = 1 << 27
 } CoglDebugFlags;
 
 #ifdef COGL_ENABLE_DEBUG