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
bnc#719994: We need the proper alpha mask

Change-Id: Ic16dceaca431929238fcef6be31ee95e068b91e4