add a way to render to canvas with DrawingContext with public api

src/Skia/Avalonia.Skia/ExternalCanvasRenderTarget.cs

@@ -0,0 +1,32 @@
+using Avalonia.Platform;
+using Avalonia.Rendering;
+
+namespace Avalonia.Skia
+{
+    internal class ExternalCanvasRenderTarget : IRenderTarget
+    {
+        private IExternalCanvasSurface _surface;
+
+        public ExternalCanvasRenderTarget(IExternalCanvasSurface canvas)
+        {
+            _surface = canvas;
+        }
+
+        public IDrawingContextImpl CreateDrawingContext(IVisualBrushRenderer visualBrushRenderer)
+        {
+            var createInfo = new DrawingContextImpl.CreateInfo
+            {
+                Canvas = _surface.Canvas,
+                Dpi = _surface.Dpi,
+                VisualBrushRenderer = visualBrushRenderer,
+                DisableTextLcdRendering = true,
+            };
+
+            return new DrawingContextImpl(createInfo);
+        }
+
+        public void Dispose()
+        {
+        }
+    }
+}

src/Skia/Avalonia.Skia/IExternalCanvasSurface.cs

@@ -0,0 +1,11 @@
+using SkiaSharp;
+
+namespace Avalonia.Skia
+{
+    public interface IExternalCanvasSurface
+    {
+        SKCanvas Canvas { get; }
+
+        Vector Dpi { get; }
+    }
+}

src/Skia/Avalonia.Skia/PlatformRenderInterface.cs

@@ -145,6 +145,8 @@ namespace Avalonia.Skia
             {
                 if (surface is IFramebufferPlatformSurface framebufferSurface)
                     return new FramebufferRenderTarget(framebufferSurface);
+                if (surface is IExternalCanvasSurface canvas)
+                    return new ExternalCanvasRenderTarget(canvas);
             }
 
             throw new NotSupportedException(