[mutter/gnome-3-34] backend-x11: Reintroduce XInitThreads

[Robert Mader <rmader src gnome org>]

commit 8b3f1117be1e90866747fa9c9e7ae1c5812ba918
Author: Daniel van Vugt <daniel van vugt canonical com>
Date:   Thu May 14 18:13:56 2020 +0800

    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 1d5f9b6917517d6e777a652cfa5e648794dc632a)

 src/backends/x11/meta-backend-x11.c | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/src/backends/x11/meta-backend-x11.c b/src/backends/x11/meta-backend-x11.c
index 225d14d33..539ca5348 100644
--- a/src/backends/x11/meta-backend-x11.c
+++ b/src/backends/x11/meta-backend-x11.c
@@ -859,6 +859,7 @@ meta_backend_x11_class_init (MetaBackendX11Class *klass)
 static void
 meta_backend_x11_init (MetaBackendX11 *x11)
 {
+  XInitThreads ();
 }
 
 Display *