fix failing test.

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

@@ -360,7 +360,7 @@ namespace Avalonia.Visuals.UnitTests.Rendering.SceneGraph
 
                 var result = initial.CloneScene();
                 sceneBuilder.Update(result, border);
-                
+
                 var borderNode = (VisualNode)result.Root.Children[0];
                 Assert.Same(border, borderNode.Visual);
 
@@ -880,16 +880,24 @@ namespace Avalonia.Visuals.UnitTests.Rendering.SceneGraph
         {
             using (UnitTestApplication.Start(TestServices.MockPlatformRenderInterface))
             {
-                var bitmap = RefCountable.Create(Mock.Of<IBitmapImpl>());
+                var bitmap = RefCountable.Create(Mock.Of<IBitmapImpl>(
+                    x => x.PixelSize == new PixelSize(100, 100) &&
+                    x.Dpi == new Vector(96, 96)));
+
                 Image img;
                 var tree = new TestRoot
                 {
                     Child = img = new Image
                     {
-                        Source = new Bitmap(bitmap)
+                        Source = new Bitmap(bitmap),
+                        Height = 100,
+                        Width = 100
                     }
                 };
 
+                tree.Measure(Size.Infinity);
+                tree.Arrange(new Rect(new Size(100, 100)));
+
                 Assert.Equal(2, bitmap.RefCount);
                 IRef<IDrawOperation> operation;