Function calculate_transition 

pub(crate) fn calculate_transition(
    source_world: &Arc<World>,
    target_world: &Arc<World>,
    entity: &dyn Entity,
    portal_entry_pos: BlockPos,
    approximate_exit_pos: BlockPos,
    to_nether: bool,
) -> Option<TeleportTransition>

Calculates a Nether portal teleport transition after the target chunks are available.