RuntimeError: dictionary changed size during iteration (When using pyglet)

Nobody has claimed this yet.

When using Pyglet instead of PyQt (which doesnt work and throws a segfault), The process finishes correctly and all blocks are exported. However, the GUI closes with the following traceback;

```
Traceback (most recent call last):
File "/home/casper/envy/bin/blockcrafter-export", line 11, in
load_entry_point('blockcrafter', 'console_scripts', 'blockcrafter-export')()
File "/home/casper/tmpedits/blockcrafter/blockcrafter/export.py", line 217, in main
app.run()
File "/home/casper/envy/lib/python3.4/site-packages/vispy/app/_default_app.py", line 62, in run
return default_app.run()
File "/home/casper/envy/lib/python3.4/site-packages/vispy/app/application.py", line 142, in run
return self._backend._vispy_run()
File "/home/casper/envy/lib/python3.4/site-packages/vispy/app/backends/_pyglet.py", line 159, in _vispy_run
return pyglet.app.run()
File "/home/casper/envy/lib/python3.4/site-packages/pyglet/app/__init__.py", line 138, in run
event_loop.run()
File "/home/casper/envy/lib/python3.4/site-packages/pyglet/app/base.py", line 142, in run
self._run()
File "/home/casper/envy/lib/python3.4/site-packages/pyglet/app/base.py", line 154, in _run
timeout = self.idle()
File "/home/casper/envy/lib/python3.4/site-packages/pyglet/app/base.py", line 278, in idle
for window in app.windows:
File "/home/casper/envy/lib/python3.4/site-packages/pyglet/app/__init__.py", line 100, in __iter__
for key in self._dict.keys():
File "/home/casper/envy/lib/python3.4/weakref.py", line 418, in keys
for wr in self.data:
RuntimeError: dictionary changed size during iteration

```

Start at blockcrafter/export.py:217, where app.run() enters the GUI event loop, then compare the Vispy and Pyglet traceback through pyglet/app/base.py and pyglet/app/__init__.py. Reproduce the export with the Pyglet backend and determine why the window collection changes during iteration; done means the GUI closes without the RuntimeError after all blocks are exported.

Written by the indexing model from the issue text.