In case of ROUNDDOWN and ROUNDUP, it is possible that seemingly clear decimal
values are rounded unexpectedly (from the user's POV). This is caused by the i
decimal to binary to decimal conversions.
By rounding to 12 significanr digits before calling the round-down of -up
function, most of these unexpected roundings are eliminated.

Change-Id: Ia19181383b77e1ff40a067c4a1cea1ece0955871
Reviewed-on: https://gerrit.libreoffice.org/69762
Tested-by: Jenkins
Reviewed-by: Eike Rathke <erack@redhat.com>