Commit 0ee361d994 regressed on
https://github.com/slint-ui/slint/issues/1269 as the inner_size()
returned at the time during event processing is already the future
value, that will change again when processing the resize event.
Instead of storing the last received value, process the event at the
appropriate MainEventsCleared stage.
Resize the glutin context when we receive a resize event from winit.
This is the approach recommended by glutin/winit's documentation.
This approach also avoids jitter when rendering with metal (not
implemented yet, but tested locally :)
This field duplicates the winit window's inner size. It is set on a
WindowEvent::Resized(), after which
`winit:🪟:Window::inner_size()` returns the same value.
