core: Set up EGL context creation thread before dropping capabilities

We do intend this thread to preserve the necessary capabilities to
create high priority EGL contexts. Set this thread up before we drop
these.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/923

src/core/main.c

@@ -591,7 +591,10 @@ meta_init (void)
 
 #ifdef HAVE_NATIVE_BACKEND
   if (backend_gtype == META_TYPE_BACKEND_NATIVE)
+    {
       meta_set_scheduler ();
+      cogl_egl_init_thread ();
+    }
 #endif
 
   g_unix_signal_add (SIGTERM, on_sigterm, NULL);