fix: union polygons after cutting meshes (CURA-966)
fix: union polygons after cutting meshes (CURA-966)

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