Create VisualNode for invisible controls.

If we don't do this, then a `VisualNode` can appear twice in its parent `ChildrenCollection` when an earlier sibling is made invisible.

Fixes #3115

src/Avalonia.Visuals/Rendering/SceneGraph/SceneBuilder.cs

@@ -240,6 +240,10 @@ namespace Avalonia.Rendering.SceneGraph
                     }
                 }
             }
+            else
+            {
+                contextImpl.BeginUpdate(node).Dispose();
+            }
         }
 
         private void UpdateSize(Scene scene)

tests/Avalonia.Visuals.UnitTests/Rendering/SceneGraph/SceneBuilderTests.cs

@@ -624,8 +624,9 @@ namespace Avalonia.Visuals.UnitTests.Rendering.SceneGraph
                 var panelNode = (VisualNode)scene.FindNode(panel);
                 sceneBuilder.Update(scene, decorator);
 
-                Assert.Equal(1, panelNode.Children.Count);
+                Assert.Equal(2, panelNode.Children.Count);
                 Assert.False(panelNode.Children[0].Disposed);
+                Assert.False(panelNode.Children[1].Disposed);
             }
         }