Handle inserts before currently visible item.

If a virtualized list was scrolled and an item added before the first
visible item, then the list wasn't updated.

src/Avalonia.Controls/Presenters/ItemVirtualizerSimple.cs

@@ -155,8 +155,7 @@ namespace Avalonia.Controls.Presenters
                     case NotifyCollectionChangedAction.Add:
                         CreateAndRemoveContainers();
 
-                        if (e.NewStartingIndex >= FirstIndex &&
-                            e.NewStartingIndex < NextIndex)
+                        if (e.NewStartingIndex < NextIndex)
                         {
                             RecycleContainers();
                         }