OpenTK in Avalonia

OpenGLControl.fs

type OpenGLControl() as this =
    inherit Control()

    let mode = new Graphics.GraphicsMode(new Graphics.ColorFormat(8, 8, 8, 8), 24, 0, 0, Graphics.ColorFormat.Empty, 1);
    let mutable win : GameWindow = null

    do 
        this.IsHitTestVisible <- false

    override this.OnAttachedToVisualTree args =
        win <- new GameWindow(640, 480, mode, "", GameWindowFlags.Default, DisplayDevice.Default, 3, 0, Graphics.GraphicsContextFlags.Default);
        win.Visible <- false
        win.MakeCurrent()

    override this.MeasureOverride size =
        win.Size <- new Size(int size.Width, int size.Height)
        size

    abstract GetFrame : unit -> unit

    default this.GetFrame () =
        GL.ClearColor(0.7f, 0.0f, 0.0f, 1.0f);
        GL.Clear(ClearBufferMask.ColorBufferBit ||| ClearBufferMask.AccumBufferBit);

    override this.Render ctx =

        this.GetFrame()   
        GL.Flush()

        use bitmap = new WriteableBitmap(
            Avalonia.PixelSize(win.Width, win.Height),
            Avalonia.Vector(96.0, 96.0),
            Nullable(Avalonia.Platform.PixelFormat.Rgba8888)
        )

        using (bitmap.Lock()) (fun buffer ->
            GL.PixelStore(PixelStoreParameter.PackRowLength, buffer.RowBytes / 4);
            GL.ReadPixels(0, 0, win.Width, win.Height, PixelFormat.Rgba, PixelType.UnsignedByte, buffer.Address);
        )        

        ctx.DrawImage(bitmap, 1.0, Avalonia.Rect(bitmap.Size), Avalonia.Rect(bitmap.Size), BitmapInterpolationMode.LowQuality)