Refresh all background instances after suspend if needed

NVIDIA drivers don't preserve FBO contents across suspend / resume
cycles which results in broken backgrounds. We can work around that by
forcing a refresh when coming out of suspend.

https://bugzilla.gnome.org/show_bug.cgi?id=739178

src/shell-util.h

@@ -45,6 +45,8 @@ GdkPixbuf *shell_util_create_pixbuf_from_data (const guchar      *data,
 void    shell_util_cursor_tracker_to_clutter (MetaCursorTracker *tracker,
                                               ClutterTexture    *texture);
 
+gboolean shell_util_need_background_refresh (void);
+
 G_END_DECLS
 
 #endif /* __SHELL_UTIL_H__ */