Just invert the scrollbar the same way gtk+ does by checking the
button positions and adjust the drawing style accordingly.

Change-Id: I3475c33e074c1509a73da618a4efa44dc61a1a77
(cherry picked from commit 273df176)
Reviewed-on: https://gerrit.libreoffice.org/18033Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>