core: Select the X11 backend of Clutter

Mutter depends on the X11 windowing backend of Clutter, unless it's used
as a Wayland display server.

This allows Mutter to run without breaking in case Clutter changes the
order with which windowing backends are selected, like it was the case
for bug https://bugzilla.gnome.org/show_bug.cgi?id=734587

The order of selection of the Clutter backends has not been made public,
so it cannot be relied upon since the introduction of the multiple
backends support; since Mutter requires the X11 backend functionality,
it should select the X11 windowing system, in the same way it selects
the EGL backend when compiled and run as a Wayland display server.

src/core/main.c

@@ -319,7 +319,9 @@ meta_init (void)
 #if defined(CLUTTER_WINDOWING_EGL) && defined(HAVE_NATIVE_BACKEND)
   if (opt_display_server)
     clutter_set_windowing_backend (CLUTTER_WINDOWING_EGL);
+  else
 #endif
+    clutter_set_windowing_backend (CLUTTER_WINDOWING_X11);
 
   meta_set_is_wayland_compositor (opt_wayland);