Considering the strict requirement that applications can't keep windows
across run_ondemand calls, this tries to make the window_ondemand example
explicitly wait for its Window to be destroyed before exiting each
run_ondemand iteration.
This updates the example to only `.set_exit()` after it gets a
`Destroyed` event after the Window has been dropped.
On Windows this works to ensure the Window is destroyed before the
example waits for 5 seconds.
Unfortunately though:
1. The Wayland backend doesn't emit `Destroyed` events for windows
2. The macOS backend emits `Destroyed` events before the window is
really destroyed.
and so the example isn't currently portable.
e5eb253698
