bnc#719994: Fix the duotone filter to actually produce duotone

Not sure what it tried to do earlier, but it was obviously wrong.

Change-Id: I0b377c4baa88205a9a7ad7c134ec5bc2223e6205
(cherry picked from commit c03f7f2b427f7fe2b94dfb55ea64670426d1a957)
Reviewed-on: https://gerrit.libreoffice.org/11127
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>