Revert "Use std::next instead of incrementing .begin()"
Revert "Use std::next instead of incrementing .begin()"

This reverts commit c85146457e924da28a19612d70d60f83687a0d1a.

Contributes to issue CURA-1112.

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