Simplify and robustify transport cargo killing and veterancy

[clyfordv]

1. When killing cargo, the instigator unit is provided. The engine then counts kills for score and unitview purposes.
2. Similarly, the mass of all cargo is distributed to the killer(s) (via lua) with VeterancyComponent.VeterancyDispersal.
3. For transports with external storage, cargo is cached so impact effects can be delayed/applied properly when the transport hits the ground.
4. Lastly, the new function explicitly kills all units (previously relied on the engine to do it), which (hopefully) addresses a bug where units who were supposed to die escaped their fate.

And hopefully all in a much more straightforward way than before!

[lL1l1]

I tested the changes with continentals of SACU or harbingers, a T1 transport with SACU/harbingers, a carrier full of strategic bombers, and a CZAR. The veterancy distribution works.
In the suggestions, I fixed two bugs I found:

• units don't detach from air transports nor spawn wrecks on transport death. They also play death effects when the transport dies.
• Units in carriers (transport internal storage) play death effects/weapons (but aircraft don't have death weapons so its less noticeable). They shouldn't do this because it looks bad.

There is another bug that I'm unsure of the best way to fix:

• transports/carriers can't be ctrl-k'd.
  This bug is caused because Entity:Kill() is separate from Entity:Kill(instigator, damageType, excessDamageRatio), and the latter is called if any 3 arguments are passed to :Kill, even if they are nil, and the arguments must be valid (instigator is Unit | nil, damageType is string, excessDamageRatio is number).
  Solutions: Add (nil, '', 0) to selfdestruct.lua:45, add if not instigator then damageType = '' excessDamageRatio = 0 end to the AirTransportUnit and AircraftCarrierUnit Kill functions, or use BaseUnitKill(self) when instigator == nil inside the unit classes and inside killcargo.

The best fix would just be whatever is most easily documented I guess.

Future? possible improvements:

• wrecks from transports' internal storages
• can an internal storage + external clamps transport exist? If so, would it break the logic?

Don't forget the changelog by the way.

[clyfordv]

Fixed ctrl-k, probably had something to do with removing the part with comments to the effect of "do not remove this part or it won't work".

[MrRowey]

Please Request a review from me once PR is Complete :)

[clyfordv]

Alright we should be solid (tested it this time!)

Improvements:

• Added TransportInternal and TransportExternal as damage types (currently only TransportInternal is relevant, so we could trim it down to just Transport if people like the simplicity).
• Reworked the logic so KillCargo only runs once on transports-in-transports.
• OnKilled for mobile units and aircraft runs a check for the TransportInternal damage type and will not play any effects, death threads, shield bounce projectiles, etc. when the internal storage they're in dies.

Possible issues:

• The divide between score/AI and visual/audio/etc. in OnKilled is not super strong, or at least not formalized. (For example, do death threads do anything that should still happen if killed inside a transport?)

Minor issues:

• Discovered that external factory dummy units are counted as losses to the player at the end of the game (2, actually--1 structure, 1 layer). This happens in deploy/fafdevelop as well.

Future ideas:

• Tangentially related, but when a transport enters a transport it could detach its unit, store it in the carrier on another bone, then retrieve it when it exits. This would require a little rework of the above, but would solve the ghost selection issue when a stinger with cargo is in a carrier.

[MrRowey]

@relent0r will any of the changes here affect ai ?

[clyfordv]

I mediated on it for a bit, trimming down to a single damage type ("TransportDamage") felt right. Let me know if there's any objections.

[lL1l1]

I tested it and found no errors.

[relent0r]

@relent0r will any of the changes here affect ai ?

I don't see anything that would impact the default AI. The changing from OnKilled to Kill might impact other AI's that are tracking deaths of units. But it looks like we are trying to make it consistent between files so makes sense.

[MrRowey]

@lL1l1 you happy with the changes so far? if so can you approve the PR so I know

[clyfordv]

killedInTransport flagging variable is renamed back, so we should be squared away