#222 make sure threads exit clean when closing pygame display

[Nathan Oostendorp]

per our google group:

If I use the code from the edge detection demo video, I sometimes
receive this error (in subject line) when closing the display. Is this
expected from that code or is there something I missed or a way to
prevent this?

I think this is just that we need a soft exit from the pygame display thread. Look at how to do this.