more layerPlanBuffer refactor and computation of where to insert the preheat command
more layerPlanBuffer refactor and computation of where to insert the preheat command

totalretribution/CuraEngine,ROBO3D/CuraEngine,markwal/CuraEngine,totalretribution/CuraEngine,alephobjects/CuraEngine,markwal/CuraEngine,pratikshashroff/pcura,totalretribution/CuraEngine,ROBO3D/CuraEngine,ROBO3D/CuraEngine,alephobjects/CuraEngine,markwal/CuraEngine,pratikshashroff/pcura,Ultimaker/CuraEngine,pratikshashroff/pcura,Ultimaker/CuraEngine,alephobjects/CuraEngine