Add observable.

Remove borders from sample to better visualize error.

samples/RenderDemo/Pages/Transform3DPage.axaml

@@ -162,24 +162,24 @@
                                   Depth="{Binding Depth}"/>
                     </Border.RenderTransform>
                 </Border>
-                <Border Classes="Test Rect2">
+                <!--<Border Classes="Test Rect2">
                     <Border.RenderTransform>
                         <Rotate3DTransform CenterZ="-100"
                                   Depth="{Binding Depth}"/>
                     </Border.RenderTransform>
-                </Border>
+                </Border>-->
                 <Border Classes="Test Rect3">
                     <Border.RenderTransform>
                         <Rotate3DTransform CenterZ="-100"
                                   Depth="{Binding Depth}"/>
                     </Border.RenderTransform>
                 </Border>
-                <Border Classes="Test Rect4">
+                <!--<Border Classes="Test Rect4">
                     <Border.RenderTransform>
                         <Rotate3DTransform CenterZ="-100"
                                   Depth="{Binding Depth}"/>
                     </Border.RenderTransform>
-                </Border>
+                </Border>-->
             </Canvas>
         </StackPanel>
         <StackPanel Grid.Row="1">

src/Avalonia.Visuals/Media/Rotate3DTransform.cs

@@ -64,6 +64,7 @@ public class Rotate3DTransform : Transform
         this.GetObservable(CenterXProperty).Subscribe(_ => RaiseChanged());
         this.GetObservable(CenterYProperty).Subscribe(_ => RaiseChanged());
         this.GetObservable(CenterZProperty).Subscribe(_ => RaiseChanged());
+        this.GetObservable(DepthProperty).Subscribe(_ => RaiseChanged());
     }
 
     /// <summary>