WebKit Scrollbars!

It was fairly easy to revive the existing Windows rendering under WebKit. Scrollbars, however, turned out to be fairly tricky. The basic problem is that unlike all other rendered objects in the frame, Scrollbars are not rendered through the usual pipeline. Instead, a separate infrastructure is in place to hook into the scrollbar controls.

Eventually, the theme logic in WebKit will be refactored such that rendering is done mostly the same way across the board, with individual 'theme' objects that do the actual work of drawing the widgets.

For now, I ended up having to duplicate logic from RenderThemeWin in PlatformScrollbarWin to load the them library and draw.

I also attempted to retain the 'legacy' windows drawing commands, though this has received no testing.