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
6db7d8f830