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

svn path=/branches/KDE/4.2/kdelibs/; revision=939470