E.g. when a Witer comment has long enough contents to get scrolled, the
desktop case repaints its contents by vcl::Window::Scroll(), followed by
a vcl::Window::Update(), i.e. vcl::Window::Invalidate() is not called.

The result of this in case of tiled rendering is that no tile is
repainted, so call vcl::Window::Invalidate() after scrolling manually.

A generic VCL-level invalidate-after-scroll is not wanted, as the
scrolling of the document contents and the main document window is not
in sync when tiled rendering.

Change-Id: I74296a0f0e52c92cf3db6aefa73f28bb2b202871