fdo#64957: #i124233# prevent the accumulation of rounding errors

in CTLayout::FillDXArry()

Change-Id: Ifb4f16fb76f67d5b5c96fdb0545c64eb8316aa3c
Reviewed-on: https://gerrit.libreoffice.org/8359
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>