Fix SF bug # 602259, 3rd parameter for Tkinter.scan_dragto

Add the optional gain parameter and pass it to Tk.
diff --git a/Misc/NEWS b/Misc/NEWS
index b9d108a..60b20d2 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -88,6 +88,9 @@
 
 - urlparse can now parse imap:// URLs.  See SF feature request #618024.
 
+- Tkinter.Canvas.scan_dragto() provides an optional parameter to support
+  the gain value which is passed to Tk.  SF bug# 602259.
+
 Tools/Demos
 -----------