win32: Use a dummy window to support delayed stage creation

The Win32 backend now implements the create_context method which
creates a context and binds it to a 1x1 invisible window. That way
there will always be a context bound and the features can be retrieved
without creating the default stage. This reflects the changes in
1c6ffc8..b245d55 to the GLX backend.

clutter/win32/clutter-backend-win32.h

@@ -46,6 +46,8 @@ struct _ClutterBackendWin32
   ClutterBackend parent_instance;
 
   HGLRC          gl_context;
+  HWND           dummy_hwnd;
+  HDC            dummy_dc;
   gboolean       no_event_retrieval;
 
   HCURSOR        invisible_cursor;