EMF+: actually use lineJoin attribute for polygons.

Change-Id: I5b369703333332598353d9817f0253bbe5fc3601
(cherry picked from commit 9b1ceabd5f696500d4fe2acb89170bd987966a9b)
Reviewed-on: https://gerrit.libreoffice.org/7248
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>