Add Tiberian Sun forest fire/tree burning logic

[henskelion]

Tiberian Sun included logic for tree flammability. This didn't just mean fire weapons damage trees, but that warheads with the "sparky" function would set trees on fire, creating a looping flame animation on the tree that would slowly damage it, while also having a random chance every frame to spread to nearby trees.

Articles on how this logic worked can be found here and here:

https://www.modenc.renegadeprojects.com/TreeFlammability

https://www.modenc.renegadeprojects.com/TreeFire

[Mailaender]

This is how I did it OpenHV/OpenHV@92f6359 in @OpenHV.

[pchote]

The TreeFlammability and TreeFire entries give a good explanation on how to implement the ignition and spread probabilities.

The damage and animation switching from big to small to none is implemented using the standard animation system with FIRE1, FIRE2, FIRE3 in art.ini that are essentially self documenting.