Reduce memory usage of constructing top surface
Reduce memory usage of constructing top surface

All of the polygons were being copied twice in this process. This skips one of the copies.

Contributes to issue CURA-3903.

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