add public api to create skia drawing context from canvas

src/Skia/Avalonia.Skia/Helpers/DrawingContextHelper.cs

@@ -0,0 +1,31 @@
+using Avalonia.Platform;
+using Avalonia.Rendering;
+using SkiaSharp;
+
+namespace Avalonia.Skia.Helpers
+{
+    public class DrawingContextHelper
+    {
+        /// <summary>
+        /// Wrap Skia canvas in drawing context so we can use Avalonia api to render to external skia canvas
+        /// this is useful in scenarios where canvas is not controlled by application, but received from another non avalonia api
+        /// like: SKCanvas canvas = SKDocument.BeginPage(...);
+        /// </summary>
+        /// <param name="canvas"></param>
+        /// <param name="dpi"></param>
+        /// <param name="visualBrushRenderer"></param>
+        /// <returns>DrawingContext</returns>
+        public static IDrawingContextImpl WrapSkiaCanvas(SKCanvas canvas, Vector dpi, IVisualBrushRenderer visualBrushRenderer = null)
+        {
+            var createInfo = new DrawingContextImpl.CreateInfo
+            {
+                Canvas = canvas,
+                Dpi = dpi,
+                VisualBrushRenderer = visualBrushRenderer,
+                DisableTextLcdRendering = true,
+            };
+
+            return new DrawingContextImpl(createInfo);
+        }
+    }
+}