pyglet windows crash on close if there's multithreading

[Jeremy Hill]

Observed with --backend=pyglet --acceleration=False; not sure whether also with --acceleration=True. Doesn't happen when IPython is started first and you run a threaded World, but does happens when there's a threaded --shell running and/or possibly when there's a matplotlib window opening.

[Jeremy Hill]

This problem is possibly related to #25 and is rarer now that (a) most of the time, the shell is not invoked with a special --shell option that triggers shell=True in AutoFinish(), but rather with the demo or shell subcommands which invoke RunShadyScript() and do not seem to have a problem, and (b) #25 is semi-fixed.