Improve performance of materials::get_rotting

[Aivean]

Summary

SUMMARY: Performance "Improve performance of materials::get_rotting"

Purpose of change

While waiting for the long time near a building with many rotting corpses, I noticed that materials::get_rotting shows up in profiler.

Describe the solution

Before materials::get_rotting was filtering the whole list of materials to find those susceptible to rot.

I reused the new versioning mechanism in generic_factory(see #44261) to create a static cache that is invalidated with corresponding generic_factory.

Then I used such cache to cache the set of materials inside the materials::get_rotting.

Describe alternatives you've considered

At first I thought of implementing "versioned cache" in the following way:

template<typename C>
    class cache {
        friend generic_factory<T>;

        int64_t& factory_verion;
        int64_t cache_verion;
        std::function<C()> init;
        C value;

        cache(int64_t &factoryVerion, const std::function<C()> &init) : factory_verion(
                factoryVerion), cache_verion(factoryVerion), init(init), value(init()) {}

    public:
        cache(const cache&) = default;
        C& get_value() {
            if (cache_verion != factory_verion) {
                value = init();
                cache_verion = factory_verion;
            }
            return value;
        }
    };


template<typename C>
cache<C> generic_factory::create_cache(std::function<C()> init) {
    return cache<C> (version, init);
}

However, I figured the reference to the generic_factory version int64_t& factory_verion; is unsafe if generic factory is not static, so I went with simpler and safer albeit less "pretty" solution.

Testing

With debugger I ensured that cache is initialized and reused correctly, and also invalidated and reinitialized when game reloads.
Also added unit tests.

Additional context

Profiling results.

Before:

After:

Note, that although the set of rotting materials is passed around by value, compiler seems to optimize that away, as the set is constant.