Add temporary cogl-clutter.h to aid splitting out Cogl

This gives us a way to clearly track the internal Cogl API that Clutter
depends on. The aim is to split Cogl out from Clutter into a standalone
3D graphics API and eventually we want to get rid of any private
interfaces for Clutter so its useful to have a handle on that task.
Actually it's not as bad as I was expecting though.

cogl/cogl-private.h

@@ -26,6 +26,9 @@
 
 G_BEGIN_DECLS
 
+gboolean
+_cogl_check_extension (const char *name, const char *ext);
+
 void
 _cogl_clear (const CoglColor *color, unsigned long buffers);