Only call super-expensive Invalidate on scrollbar toggling

otherwise even using backspace in an annotated area will cause super slow
behaviour as each keystroke causes a full page render

This became a problem after

commit 0761f81643a6890457e9ef7d913ab5c88c2593a4
Date:   Thu Dec 19 18:50:58 2013 +0000
    123792: complete annotations on text ranges feature

but underlying problem was always there ready to trigger.

For this case only render the full page if the state
of comments scrollbars *toggles*, i.e. if there wasn't
scrollbars and there ends up still with no scrollbars
avoid the (bad) hack of invalidating the page

(cherry picked from commit 378c74ce290ab772f962db51ff8cba200cb40144)

Conflicts:
	sw/source/uibase/docvw/PostItMgr.cxx

Change-Id: Ic7fd432d2317b0f5ad8d8773636f99fe75d660d6
(cherry picked from commit 7e75eab406c5e58e40987ab8a249dbacbb685551)
Reviewed-on: https://gerrit.libreoffice.org/13279
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>