When scrolling part of the window, scroll the update region as well so that

parts that haven't been painted yet still get an update event in their new
location.  This code seems far too complicated, and I suspect there's a
better way.  Still, scrolling continuously with the window partially
off-screen now works.

git-svn-id: svn://svn.tartarus.org/sgt/putty@2284 cda61777-01e9-0310-a592-d414129be87e