Client Car Spawning

[leonskij]

I checked out #112, but it did not work in my testing. Thus, I thought I would have a crack at it. If #112 does indeed work then feel free to close this.

Allows cars spawned via the client's CarSpawner.SpawnCar() and CarSpawner.SpawnCars() methods to be correctly requested to be spawned on the server from the client.

There are improvements that could be made, such as possibly linking the client-spawned car to the networked car instead of despawning it just for the server to re-create it. But, I'm new to the code-base and still getting used to it.

[AMacro]

Hey @leonskij,

Haven't forgotten this PR, still need to test & review #112 and this one.

In general everything should be server authorative (rerailing and clearing cars from the comms radio is an example), where the client requests an action and the server either accepts and processes or rejects the action.

In the case of the spawn workflow I would expect:

1. Client requests a spawn of xyz car at location x
2. Server validates the request (is the player close enough to the spawn location, is there enough money, is the player authorised to perform the action, etc.
3. If the server accepts the request, spawn the car at the desired location. The CarSpawner.SpawnCars() patch will see the spawn and send a spawn packet to all users.

There are a few traps with some client spawns, inparticular, the Work Trains, as only 1 of each type exists in the game so calling it while someone else is using it is problematic and requires validation/blocking. From memory another issue is the way it can jump across the map requires special handling to stop the physics from freaking out.

These issues were why the spawn features are deliberately disabled until higher priority issues could be resolved, so very much hoping either your PR or #112 works :).