Also minimize rounding errors in tab stop pixel positions.

Change-Id: I929d8ca0cd4fc665ea66bf47a7e3b5e47dc8e7f2
Reviewed-on: https://gerrit.libreoffice.org/20762Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Oliver Specht <oliver.specht@cib.de>