Split stacking context fragments into sections in layout_2020

This allows rendering stacking context content in the proper order. We
also need to add a new pseudo stacking context for atomic inlines in
order to preserve proper rendering order in some cases.

components/layout_2020/flow/root.rs

@@ -191,7 +191,7 @@ impl FragmentTreeRoot {
             );
         }
 
-        stacking_context.sort_stacking_contexts();
+        stacking_context.sort();
         stacking_context.build_display_list(builder);
     }