Add weather to shaders

[alandtse]

Some potential starting points:
Per RiverwoodModder

This should be how to calculate it, I'm just not sure where in CS to put it, and how to make it available in lighting.hlsl
    // Papyrus Weather.GetClassification 
    int32_t GetClassification(TESWeather * weather)
    {
        const auto flags = *((byte *)weather + 0x66F);

        if (flags & 1)
            return 0;
        if (flags & 2)
            return 1;
        if (flags & 4)
            return 2;
        if (flags & 8)
            return 3;

        return 0xFFFFFFFF;
    }

    bool GetCurrentWeatherClassification(int32_t &classification)
    {
        const auto skyPtr = *g_skyPtr;
        if (skyPtr && skyPtr->currentWeather)
        {
            classification = GetClassification(skyPtr->currentWeather);
            return true;
        }
        return false;
    }

[alandtse]

Some potential work started here. https://github.com/ffarrell17/skyrim-community-shaders/tree/WeatherTOD