support CSS3's background-attachment: local
cf. http://www.w3.org/TR/css3-background/#background-attachment
Scrolling the background with the element content is actually the
behaviour we were using for background-attachment: scroll, in order to
emulate MSIE6/7, but that was a violation of CSS 2.1 (#185631),
so fix that too.
(also fix bogus borderTop addition in isPointInScrollbar, noticed while
playing with thick bordered background-attachment testcases)
BUG: 185631
git-svn-id: svn://anonsvn.kde.org/home/kde/trunk/KDE/kdelibs@939169 283d02a7-25f6-0310-bc7c-ecb5cbfe19da