This fixes an issue where state bleed from the parent to the child

Interesting, I didn't know about the second argument on `Object.create`. This is probably clearer. As discussed in person, maybe something like this, that modifies the prototype -- may be more compatible with the 'run-time subclassing' that we do with stylesheet-defined modes. It also avoids the baroque syntax of `defineProperty`.

```
var Polygons = Object.create(Object.assign({}, RenderMode, {
    init(): {
        ...
    }
}));
```