backend: Add a backend release function

Mutter still relies heavily on singletons such as its MetaBackend.

For that, the backend implementation has a meta_init_backend() function
which is called at startup from meta_init(), which creates the desired
backend and sets the singleton which is returned by meta_get_backend().

Unfortunately, that means that the backend is never actually freed, and
all the code from the backend finalize function never actually get
called.

Add a meta_release_backend() to free the backend singleton.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1438

src/backends/meta-backend-private.h

@@ -108,6 +108,7 @@ struct _MetaBackendClass
 };
 
 void meta_init_backend (GType backend_gtype);
+void meta_release_backend (void);
 
 #ifdef HAVE_WAYLAND
 MetaWaylandCompositor * meta_backend_get_wayland_compositor (MetaBackend *backend);

src/backends/meta-backend.c

@@ -1393,6 +1393,12 @@ meta_init_backend (GType backend_gtype)
     }
 }
 
+void
+meta_release_backend (void)
+{
+  g_clear_object (&_backend);
+}
+
 /**
  * meta_is_stage_views_enabled:
  *