Improve vehicle spawning

[mole99]

Currently when vehicles spawn in traffic, there are some problems that can occur.

Some of them are:

• [ ] Vehicles can spawn in view (problem with viewing frustum)
• [ ] Vehicles spawn in groups i.e. too close to each other
• [ ] The type of the vehicle doesn't correspond with the current zone
• [ ] The type of the driver doesn't depend upon the vehicle type

Concerning the vehicle and driver types, @SonofUgly wrote:

There's a section in default.ide that has the spawn frequency and puts the vehicles into classes for where they should spawn.

[SonofUgly]

Documenting GTA 3 car spawns per district should be quite useful.