Fix time estimates with accelerations
Fix time estimates with accelerations

This was giving NaN values in the time estimates for the edge case where the discriminant of this quadratic was actually 0.0, but due to floating point rounding errors ended up below 0.

Contributes to issue CURA-1717.

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