Optimise bulk construction of ScMarkArray.

This takes the opening time from 46s to 40.5s for me.

Change-Id: I3955fe9b2c3113dac2ae3cda97d692de1975e762
Reviewed-on: https://gerrit.libreoffice.org/72418
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>