The preview is now drawn onto it's own window rather directly onto
the client area of the top level window. This is done to allow for
scrollbars, since the page won't usually fit in the window when zoomed in.
Refactoring was needed as a result of this, and in order to provide more
granularity for updates to avoid EM_FORMATRANGE messages being sent when
the page the same page is being displayed.
Aurimas Fišeras