Added failing custom renderer test.

Deferred rendering does not yet support opacity from `Visual.Render`.

tests/Avalonia.RenderTests/Avalonia.RenderTests.projitems

@@ -9,6 +9,7 @@
     <Import_RootNamespace>Avalonia.RenderTests</Import_RootNamespace>
   </PropertyGroup>
   <ItemGroup>
+    <Compile Include="$(MSBuildThisFileDirectory)Controls\CustomRenderTests.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)GeometryClippingTests.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Media\RadialGradientBrushTests.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Media\BitmapTests.cs" />

tests/Avalonia.RenderTests/Controls/CustomRenderTests.cs

@@ -0,0 +1,59 @@
+// Copyright (c) The Avalonia Project. All rights reserved.
+// Licensed under the MIT license. See licence.md file in the project root for full license information.
+
+using System.Threading.Tasks;
+using Avalonia.Controls;
+using Avalonia.Layout;
+using Avalonia.Media;
+using Xunit;
+
+#if AVALONIA_CAIRO
+namespace Avalonia.Cairo.RenderTests.Controls
+#elif AVALONIA_SKIA
+namespace Avalonia.Skia.RenderTests
+#else
+namespace Avalonia.Direct2D1.RenderTests.Controls
+#endif
+{
+    public class CustomRenderTests : TestBase
+    {
+        public CustomRenderTests()
+            : base(@"Controls\CustomRender")
+        {
+        }
+
+        [Fact]
+        public async Task Opacity()
+        {
+            Decorator target = new Decorator
+            {
+                Padding = new Thickness(8),
+                Width = 200,
+                Height = 200,
+                Child = new OpacityRenderer(),
+            };
+
+            await RenderToFile(target);
+            CompareImages();
+        }
+
+        class OpacityRenderer : Control
+        {
+            public override void Render(DrawingContext context)
+            {
+                context.FillRectangle(
+                    Brushes.Red,
+                    Bounds,
+                    4);
+
+                using (context.PushOpacity(0.5))
+                {
+                    context.FillRectangle(
+                        Brushes.Blue,
+                        Bounds.Deflate(20),
+                        4);
+                }
+            }
+        }
+    }
+}