`Proxy` is the task queue as well as the way an application sends its own
events, so `ProxyQueue` is gone. `schedule_redraw` becomes
`request_redraw_if_needed`, which says what the comment beside it was
saying.
`assert_corners!` replaces the region helper, so `tests/common` goes with
it, and the scroll test now states both corners rather than one number.
`WakeTaskQueue` becomes `TaskQueue`, which carries the update itself.
Delivery and waking are then one act: the winit host sends it through the
`EventLoopProxy` as a `DefaultEvent::Update`, so there is no channel
beside the loop and nothing has to claim a redraw is needed in order to be
looked at. `Window::request_redraw` is gone from this path; `event`
applies the update and then asks the tree whether anything became dirty,
which is the same question `window_event` already ended with -- now
`schedule_redraw`, called from both.
The loop's message type is `DefaultEvent<State>`, so `Proxy` becomes a
wrapper that takes the application's own `Event` and requires it to be
`Send`, since it now crosses to the task thread by that route.
The harness supplies a channel-backed queue, which is what lets a test
hold updates until it asks for them.
Tests split by subject -- layout, pointer, scroll, tasks -- with the
region helper in `tests/common`.
iris