This fixes the vertical rendering artifacts visible on the bug
screenshot with the oxygen-gtk Gtk theme, and also is a lot simpler than
what we did here before.

Change-Id: I21a167452f14ae52bd0d899b3ed467ce40540dec
Reviewed-on: https://gerrit.libreoffice.org/17631Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>
(cherry picked from commit 4532ea42)
Reviewed-on: https://gerrit.libreoffice.org/17643Reviewed-by: Tor Lillqvist <tml@collabora.com>
Tested-by: Tor Lillqvist <tml@collabora.com>