Image blending operation 'soft-burn' (c = (cA+cB<1.0)?0.5*cB / (1.0 - cA):1.0-0.5*(1.0 - cA) / cB)
Irudi nahasketaren eragiketa: 'erredura leuna' (c = (cA+cB<1.0)?0.5*cB / (1.0 - cA):1.0-0.5*(1.0 - cA) / cB)