Prevent disposing on wrong threads

[SupinePandora43]

1. Set IsDisposed on this to true
2. GC.SupressFinalize(this)
3. Add (Type, Handle) to ThreadLocal<List<Type, nuint>> ToDispose

Checkthread function:

if(!ToDispose.Value.IsEmpty){
    for((type, ptr) in ToDispose.Value)
        type switch
        {
            typeof(View) => Methods.ulDestroyView((Handle<View>)ptr),
            _ => throw new OutOfRangeException()
        };
    ToDispose.Value.Clear(); // make sure it cleans up internal array
}

[SupinePandora43]

Multithreading improvements promised to come in ultralight 1.4

[SupinePandora43]

They didn't come. (at least, in readable in the blog post form)