Bug 1717351 - Enable Wayland multi-buffer software backend by default, r=stransky

As it gives us better guarantees about correctness, it allows us to enable
performance optimizations such as D118192, which reduces redraws.

Another advantage is that it does not need any extra knowledge about
compositors as it does not make any assumptions about buffer holding
behaviour.

Differential Revision: https://phabricator.services.mozilla.com/D119668

modules/libpref/init/StaticPrefList.yaml

@@ -11665,7 +11665,7 @@
 
 - name: widget.wayland.multi-buffer-software-backend.enabled
   type: bool
-  value: false
+  value: true
   mirror: once
 
 # Use opaque region for MozContainer wl_surface