just use a std::vector<XPolygon>

Change-Id: I1adb801faa8341e0759a86fd209a530b8f5c5a1c
Reviewed-on: https://gerrit.libreoffice.org/43086Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>