[mutter] main: Release backend on teardown

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit 98df888f039697418c6a7ba2fca092d08bbd47c9
Author: Olivier Fourdan <ofourdan redhat com>
Date:   Fri Sep 11 12:20:23 2020 +0200

    main: Release backend on teardown
    
    This allows to call the backend finalize function on teardown.
    
    https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1438

 src/core/main.c | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/src/core/main.c b/src/core/main.c
index 2a7cb4eafd..06605c5e22 100644
--- a/src/core/main.c
+++ b/src/core/main.c
@@ -312,6 +312,8 @@ meta_finalize (void)
   if (meta_is_wayland_compositor ())
     meta_wayland_finalize ();
 #endif
+
+  meta_release_backend ();
 }
 
 static gboolean