Get Window handle (IntPtr) from Selenium webdriver's current window GUID

I'm trying to capture a screenshot of whole browser screen (e.g. with any toolbars, panels and so on) not only an entire page, so I'm got this code:

using (FirefoxDriver driver = new FirefoxDriver())
{ 
    driver.Navigate().GoToUrl(url);                

    ScreenCapture sc = new ScreenCapture();

    // How can I find natural IntPtr handle of window here, using GUID-like identifier returning by driver.currentWindowHandle?
    Image img = sc.CaptureWindow(...);
    MemoryStream ms = new MemoryStream();
    img.Save(ms, ImageFormat.Jpeg);
    return new FileStreamResult(ms, "image/jpeg");
}