Document and test macOS Metal support

Smoke/AvaloniaStaticLinkSmoke/MainWindow.axaml.cs

@@ -1,11 +1,60 @@
+using Avalonia;
 using Avalonia.Controls;
+using Avalonia.Media;
+using Avalonia.Rendering.SceneGraph;
+using Avalonia.Skia;
 
 namespace AvaloniaStaticLinkSmoke;
 
 public sealed partial class MainWindow : Window
 {
-    public MainWindow()
+    private readonly Action<string>? _renderResult;
+    private int _renderResultReported;
+
+    public MainWindow() : this(null)
     {
+    }
+
+    public MainWindow(Action<string>? renderResult)
+    {
+        _renderResult = renderResult;
         InitializeComponent();
     }
+
+    public override void Render(DrawingContext context)
+    {
+        base.Render(context);
+        if (_renderResult is not null)
+        {
+            context.Custom(new MetalBackendProbe(new Rect(Bounds.Size), ReportMetalResult));
+        }
+    }
+
+    private void ReportMetalResult(string backend)
+    {
+        if (Interlocked.Exchange(ref _renderResultReported, 1) == 0)
+        {
+            _renderResult?.Invoke(backend);
+        }
+    }
+
+    private sealed class MetalBackendProbe(Rect bounds, Action<string> report) : ICustomDrawOperation
+    {
+        public Rect Bounds { get; } = bounds;
+
+        public void Dispose()
+        {
+        }
+
+        public bool Equals(ICustomDrawOperation? other) => false;
+
+        public bool HitTest(Point point) => false;
+
+        public void Render(ImmediateDrawingContext context)
+        {
+            var feature = context.TryGetFeature(typeof(ISkiaSharpApiLeaseFeature)) as ISkiaSharpApiLeaseFeature;
+            using var lease = feature?.Lease();
+            report(lease?.GrContext?.Backend.ToString() ?? "Software");
+        }
+    }
 }