Moves all GLX code down from Clutter to Cogl

This migrates all the GLX window system code down from the Clutter
backend code into a Cogl winsys. Moving OpenGL window system binding
code down from Clutter into Cogl is the biggest blocker to having Cogl
become a standalone 3D graphics library, so this is an important step in
that direction.

clutter/clutter-backend.c

@@ -53,6 +53,7 @@
 #include "clutter-version.h"
 
 #include <cogl/cogl.h>
+#include <cogl/cogl-internal.h>
 
 G_DEFINE_ABSTRACT_TYPE (ClutterBackend, clutter_backend, G_TYPE_OBJECT);