Shifting raft surface only for first layer

Shifting only the same layer off the raft surface is more efficient and makes the first layer cleaner

I update my recommended config for raft surface:

```
raftBaseThickness = 300
raftBaseLinewidth = 1000
raftLineSpacing = 3000
raftBaseSpeed = 8

raftInterfaceThickness = 270
raftInterfaceLinewidth = 400
raftInterfaceLineSpacing = 1000

raftSurfaceThickness = 270
raftSurfaceLinewidth = 400
raftSurfaceLineSpacing = 400
raftSurfaceLayers = 2
raftAirGap = 230
```

Seems to works good