Currently user needs to kill the process to terminate nes-rust in standalone mode. We should add a proper way to stop.

Describe the bug

Calling render from a new thread after first calling it from another results in an error (see console output below). This may well be a limitation of Pyglet, but I don't know. Note that running the environments without rendering to screen works as expected, but for some purposes (debugging of graphical glitches etc.) it can be very useful to monitor multiple environments at t