shell/global: Add 'backend' property

Replace all Meta.get_backend() with global.backend. Maybe sooner or
later we can further decrease the amount of singletons handled by
libmutter.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1210

js/ui/main.js

@@ -802,7 +802,7 @@ function showRestartMessage(message) {
 
 var AnimationsSettings = class {
     constructor() {
-        let backend = Meta.get_backend();
+        let backend = global.backend;
         if (!backend.is_rendering_hardware_accelerated()) {
             St.Settings.get().inhibit_animations();
             return;