Add SkiaPlugin support and benchmark plugins for SkiaSharp and GDI+

src/DesktopMagicPluginAPI/SkiaAsyncPlugin.cs

@@ -0,0 +1,31 @@
+using SkiaSharp;
+
+using System.Drawing;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace DesktopMagic.Api;
+
+/// <summary>
+/// Provides an abstract base class for creating asynchronous plugins using SkiaSharp rendering.
+/// Plugins override <see cref="MainAsync(SKCanvas, CancellationToken)"/> instead of <see cref="Plugin.Main()"/>.
+/// All existing <see cref="Plugin"/>, <see cref="AsyncPlugin"/>, and <see cref="SkiaPlugin"/> code is unaffected.
+/// </summary>
+public abstract class SkiaAsyncPlugin : AsyncPlugin
+{
+    /// <summary>
+    /// This method is sealed and returns null for async Skia plugins.
+    /// Override <see cref="MainAsync(SKCanvas, CancellationToken)"/> instead.
+    /// </summary>
+    public sealed override Task<Bitmap?> MainAsync(CancellationToken cancellationToken)
+    {
+        return Task.FromResult<Bitmap?>(null);
+    }
+
+    /// <summary>
+    /// Called every <see cref="Plugin.UpdateInterval"/> milliseconds to render the plugin asynchronously.
+    /// </summary>
+    /// <param name="canvas">The SkiaSharp canvas to draw on.</param>
+    /// <param name="cancellationToken">Token signaled when the host requests cancellation.</param>
+    public abstract Task MainAsync(SKCanvas canvas, CancellationToken cancellationToken);
+}