Implement request_draw

[almarklein]

Closes #63

• For qt this is easy, because it's an alias for update().
• For glfw it's more involved, because glfw does not provide an event loop. So I've made a function that one must call from your glfw mainloop, which will redraw all canvases that are invalidated.
• Updated the glfw tests (they check that multiple events or draw requests result in a single draw)
• The glfw example has a more sophisticated event loop that does not turn your laptop into a heater,

[Korijn]

LGTM