Unintuitive ceg tag behaviour for spawned cannon projectiles

If you use `Spring.SpawnProjectile` to spawn a projectile of weapontype `Cannon` it will not draw its CEG trail. This is because Cannon only draws when `ttl > 0` and SpawnProjectile defaults to infinite ttl which is negative.

https://github.com/beyond-all-reason/spring/blob/2deba2827e8dc52e694700bd14895c4d2509553b/rts/Sim/Projectiles/WeaponProjectiles/ExplosiveProjectile.cpp#L52-L53

I think `SpawnProjectile` defaulting to infinite is good. I think it would be good if cannon just drew the trail regardless of TTL but I don't know if that would make things look bad visually near expiration. Perhaps draw if `ttl != 0` so it doesn't impact "real" projectiles with positive TTL (since they explode when they reach 0) without affecting infinite negative TTL.

Start with ExplosiveProjectile.cpp at the linked TTL check and trace how Spring.SpawnProjectile supplies an infinite TTL for Cannon projectiles. Reproduce a spawned Cannon projectile and verify the CEG trail behavior near expiration. Done means infinite-TTL Cannon projectiles draw their trail without changing the behavior of regular positive-TTL projectiles.

Written by the indexing model from the issue text.