GRF loading stages:
  The stages are processed after each other. All GRFs are processed for the same stage before advancing.

  # Initialiation stage
  for newgrf in configured_newgrfs:
     # Each NewGRF peforms checks on its own wrt. its parameters and general game variables (like climate).
     if newgrf.is_happy_with_parameters():
        # Shout out "hello" to other GRFs.
     else:
        # disable newgrf

  # Reservation stage
  for newgrf in configured_newgrfs:
     # Each NewGRF may check what other GRFs called "hello", and may choose to leave the room if they don't like them,
     # and the other one did not leave again on its own.
     if newgrf.is_happy_with_other_grfs():
        # Make promises what this NewGRF provides wrt. shared items, like cargo and railtype definitions.
        newgrf.provide_cargos()
        newgrf.provide_railtypes()
     else:
        # disable newgrf

  # Activation stage
  for newgrf in configured_newgrfs:
     # Each NewGRF provides all the stuff, sometimes depending on what cargos and railtypes were promised by other GRFs.
     # Erroring out here is considered rude.
     newgrf.provide_stuff(available_cargotypes, available_railtypes)