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.

cogl/cogl-debug.h

@@ -63,6 +63,7 @@ typedef enum {
   COGL_DEBUG_DISABLE_PROGRAM_CACHES,
   COGL_DEBUG_DISABLE_FAST_READ_PIXEL,
   COGL_DEBUG_CLIPPING,
+  COGL_DEBUG_WINSYS,
 
   COGL_DEBUG_N_FLAGS
 } CoglDebugFlags;