avoid copy when adapting polygon in SdrPathObj::ImpForceKind()

Change-Id: Ifd830ecd181563102ee269c6a609ef8ebeab670c
Reviewed-on: https://gerrit.libreoffice.org/61767
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>