public override void AddColorStop(object backend, double position, Xwt.Drawing.Color color)
{
Cairo.Gradient g = (Cairo.Gradient)backend;
g.AddColorStop(position, color.ToCairoColor());
}
public static void AddColorStop(this Cairo.Gradient grad, double offset, Color c)
{
grad.AddColorStop(offset, new Cairo.Color(c.R, c.G, c.B, c.A));
}