OffScreen - ScreenshotAsync now correctly passes through blend to ScreenshotOrNull

Author: amaitland

CefSharp.OffScreen/ChromiumWebBrowser.cs

@@ -491,7 +491,7 @@ public Task<Bitmap> ScreenshotAsync(bool ignoreExistingScreenshot = false, Popup
                     // Chromium has rendered.  Tell the task about it.
                     Paint -= paint;
 
-                    completionSource.TrySetResultAsync(ScreenshotOrNull());
+                    completionSource.TrySetResultAsync(ScreenshotOrNull(blend));
                 };
 
                 Paint += paint;