ENH: Fullscreen

Closes #387.

I have only changed one example (`text_visual.py`), assuming people are happy with the other changes, I'll change all the other examples.
Not 100% sure -- I think of it as the opposite of `_vispy_warmup` (which are generally between 0.25 and 0.4 seconds), to ensure the window and context actually gets destroyed. I figured better safe than sorry for testing (where the context managers are used), but I can remove it if you think it's excessive.