Make draft shield limit work with small models
Make draft shield limit work with small models

Problem was that the draft shield height limit was measured in number of layers, but it was compared with the height of the first layer to determine if we should skip adding draft shield layers, and the first layer height was measured in microns.

Contributes to issue CURA-1295.

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