main: Expose hooks to manage context with more granularity

This is to allow gnome-shell to tear down more gracefully. It's a stop
gap solution until there is a better context handling system, but will
have to do for now.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1775>

src/meta/main.h

@@ -75,5 +75,16 @@ void meta_quit (MetaExitCode code);
 META_EXPORT
 void            meta_test_init               (void);
 
+META_EXPORT
+void meta_start (void);
+
+META_EXPORT
+void meta_run_main_loop (void);
+
+META_EXPORT
+void meta_finalize (void);
+
+META_EXPORT
+MetaExitCode meta_get_exit_code (void);
 
 #endif