Add render method with same signature as SB3 underlying class

[elliottower]

This PR fixes the render() method not matching the signature as SB3's VecEnvWrapper class, fixing #223

[elliottower]

This doesn't affect anything really, it just allows you to use the proper syntax that SB3 expects, and if you happen to have ansi or rgb_array as your underlying render_mode and your environment's render method is defined to return a value, then that will be returned to SB3, otherwise nothing happens.