shell-util: Add API to check for X11 extensions

Will be used to disable animations when running inside Xvnc. This was
done in gsd-xsettings before.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/757

src/shell-util.h

@@ -66,6 +66,9 @@ gboolean shell_util_stop_systemd_unit  (const char  *unit,
 
 void shell_util_sd_notify (void);
 
+gboolean shell_util_has_x11_display_extension (MetaDisplay *display,
+                                               const char  *extension);
+
 G_END_DECLS
 
 #endif /* __SHELL_UTIL_H__ */