fdo#82067 RTF import: handle negative SHPLEFT / SHPTOP

(cherry picked from commit dc0149869bc0cc09e92816ae6f34831bf5353e3b)

Change-Id: Id82d6d54b160b5b78a53c526e6ccab0514c21312
Reviewed-on: https://gerrit.libreoffice.org/11283
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>