Look for a whole column at once so we don't have to iterate all draw objects
for every cell, but only once per column.

Follow-up for 3a2a430a

Change-Id: Ic8740fca7d595528785b432c1cedf4fad4f13ba1
Reviewed-on: https://gerrit.libreoffice.org/48416Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Eike Rathke <erack@redhat.com>