The idea that redraw events are dispatched with a specific ordering
that makes it possible to specifically report when we have finished
dispatching redraw events isn't portable and the way in which we
dispatched RedrawEventsCleared was inconsistent across backends.
More generally speaking, there is no inherent relationship between
redrawing and event loop iterations. An event loop may wake up at any
frequency depending on what sources of input events are being listened
to but redrawing is generally throttled and in some way synchronized
with the display frequency.
Similarly there's no inherent relationship between a single event loop
iteration and the dispatching of any specific kind of "main" event.
An event loop wakes up when there are events to read (e.g. input
events or responses from a display server / compositor) and goes back
to waiting when there's nothing else to read.
There isn't really a special kind of "main" event that is dispatched
in order with respect to other events.
What we can do more portably is emit an event when the event loop
is about to block and wait for new events.
In practice this is very similar to how MainEventsCleared was
implemented except it wasn't the very last event previously since
redraw events could be dispatched afterwards.
The main backend where we don't strictly know when we're going to
wait for events is Web (since the real event loop is internal to
the browser). For now we emulate AboutToWait on Web similar to how
MainEventsCleared was dispatched.
In practice most applications almost certainly shouldn't care about
AboutToWait because the frequency of event loop iterations is
essentially arbitrary and usually irrelevant.
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.
A minimal example that shows an application running the event loop more
than once via `run_ondemand`
There is a 5 second delay between each run to help highlight problems
with destroying the window from the first loop.
