Offset all layers by the minimum layer count. This makes raft just a
minor special case when processing layers instead of needing to adjust
everything.
Contributes to CURA-653
This is done to prevent the very large messages that would be sent otherwise.
Protobuf can't do anything with messages above 512MB. As we no longer send a
"collection" message, this should no longer occur.
CURA-1210