EndTransport should use passengerID as arg

Consider editing unitScript function EndTransport to use passengerID as argument.
This will enable customized animations instead of a default unload animation.

Additionnally, consider having EndTransport(passengerID) be called for each individual unit, rather than only once the transport is empty for UnloadLandFlood and UnloadLand methods; similar to what UnloadDrop method does; especially since we can now give individual UnloadUnit commands based on specific unitIDs rather than unloading all units.
You could still find a way to call it i.e. with a bool IsEmpty value in case the final EndTransport call (the one called when all units successfully unloaded) needs to be identified for further animating.
And last, consider adding a bool "killed" arg and calling EndTransport(passengerID, killed) upon passenger dying during transport. Allowing an animation to occur specifically when a passenger died.

nb. this might need to fix the fact that passengerID can be a number but not point to any valid unitID anymore when this is called near a unit death.

Trace the unitScript EndTransport function and the UnloadLandFlood, UnloadLand, and UnloadDrop entry points first. Compare when each method unloads individual units, handles an empty transport, and reacts to a passenger death. Done means the relevant calls consistently provide the needed passenger identity and distinguish final unloading and death cases without relying on invalid unit IDs.

Written by the indexing model from the issue text.