Optionally prime on the interface and surface layers of the raft
Optionally prime on the interface and surface layers of the raft

If this is the first time the extruders are being used, prime them.

The surface layer was getting the surface_extruder_nr as the starting extruder even if the previous layer was a different extruder, so that is also fixed.

Contributes to issue CURA-8960.

Ultimaker/CuraEngine,Ultimaker/CuraEngine