fix: writeRetraction didn't unretract if already retracted more (CURA-959)
fix: writeRetraction didn't unretract if already retracted more (CURA-959)

totalretribution/CuraEngine,Ultimaker/CuraEngine,alephobjects/CuraEngine,ROBO3D/CuraEngine,alephobjects/CuraEngine,alephobjects/CuraEngine,ROBO3D/CuraEngine,Ultimaker/CuraEngine,totalretribution/CuraEngine,ROBO3D/CuraEngine,totalretribution/CuraEngine