backend-x11: Reintroduce XInitThreads

It was removed in 3.34 as part of 6ed5d2e2. And we thought that was the
only thread that might exist and use X11. But the top gnome-shell crasher
in 3.36 seems to suggest otherwise.

We don't know what or where the offending thread is, but since:

 1. We used XInitThreads for years already prior to 3.34; and

 2. Extensions or any change to mutter/gnome-shell could conceivably use
    threads to make X calls, directly or indirectly,

it's probably a good idea to reintroduce XInitThreads. The failing assertion
in libx11 is also accompanied by a strong hint:

```
fprintf(stderr, "[xcb] Most likely this is a multi-threaded client " \
                "and XInitThreads has not been called\n");
```

https://bugs.launchpad.net/bugs/1877075

Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/1252

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1256

(cherry picked from commit 1d5f9b6917)

src/backends/x11/meta-backend-x11.c

@@ -855,6 +855,7 @@ meta_backend_x11_class_init (MetaBackendX11Class *klass)
 static void
 meta_backend_x11_init (MetaBackendX11 *x11)
 {
+  XInitThreads ();
 }
 
 Display *