Fix updating current E value
Fix updating current E value

It should be updated before we use it to write the retraction. Also we should subtract the retraction because it is the amount to retract, not the amount to prime.

Contributes to issue CURA-2795.

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