add return to render method

[51616]

This allows mode='rgb_array' and 'depth_array' to return the array as in the original single-agent mujoco gym env. These modes are faster than mode='human'.

[fyqqyf]

Oh, if I had seen your request earlier, it would have saved me one day’s time (I found this problem and submitted an issue, and then I saw your request, lol). By the way, will this change cause more problems?

[51616]

@fyqqyf As far as I know, it doesn't cause any problem and you can get the rgb image from the render method.