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-framebuffer-private.h

@@ -113,6 +113,10 @@ typedef struct _CoglOnscreen
 void
 _cogl_framebuffer_state_init (void);
 
+void
+_cogl_framebuffer_winsys_update_size (CoglFramebuffer *framebuffer,
+                                      int width, int height);
+
 void
 _cogl_clear4f (unsigned long buffers,
                float red,