Fix facepalm in ItemsSourceView.

Don't construct a new `List` unless the source isn't already an `IList`.

src/Avalonia.Controls/Repeater/ItemsSourceView.cs

@@ -35,9 +35,11 @@ namespace Avalonia.Controls
         {
             Contract.Requires<ArgumentNullException>(source != null);
 
-            _inner = source as IList;
+            if (source is IList list)
-
+            {
-            if (_inner == null && source is IEnumerable<object> objectEnumerable)
+                _inner = list;
+            }
+            else if (source is IEnumerable<object> objectEnumerable)
             {
                 _inner = new List<object>(objectEnumerable);
             }