we know that ScPatternAttr uses a single, contiguous range of
item ids, so we can compute the item offset at compile time.
Shaves 1-2% off some workloads.
Change-Id: I623b8cb3e0d5d070118117196d2b48575f505725
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/165550
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>