if we are a hairline along the very right/bottom edge
of the canvas then distory the polygon inwards one pixel right/bottom so that
the hairline falls inside the paintable area and becomes visible

Change-Id: Ie5713f6916cf5b47fdf14f86f034e38cda9900fd
Reviewed-on: https://gerrit.libreoffice.org/34243Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>