Window destruction

[dhardy]

Symptom: the shello example runs fine, but hangs on exit.

Apparent cause: the X11 backend destroys a window before user state (handler) gets destroyed. This is potentially a problem because that user state may include stuff which depends on the window. At least, I appear to be hitting this issue:

• https://gitlab.freedesktop.org/mesa/mesa/-/issues/176
• (which I've hit before)

So to fix this, naively, we need to destroy the RenderSurface before the window, like this:

impl WindowState {
    // ...
    fn destroy(&mut self) {
        drop(self.surface.take());

        Application::global().quit()
    }
}

... but there are two problems here:

1. We rely on user applications to handle destruction in the correct order, despite this being a cross-platform API and the requirement only being present on one platform (X11)
2. WGPU doesn't actually destroy the swapchain here (I'm not even sure if it is supposed to, though the Surface is the only part which is window-specific)

The alternative would be to keep the X11 window alive until after user state (handler) has been destroyed. This can't simply be done from backend::x11::window::Window::destroy since self.handler is borrowed when this method is called so further redesign would be needed (e.g. ensure the user state is destroyed immediately after handle_destroy_notify is called).

There is a further complication in that user state may hold a copy of a WindowHandle, which contains a Weak reference to the window object. I wonder whether further design changes are sensible:

• Pass handle: &WindowHandle into all methods of WinHandler, remove Clone impl
• Application::clipboard returns a temporary (non-Clone) object Clipboard<'_>

[jneem]

Looks like this will be fixed in new xservers: https://gitlab.freedesktop.org/mesa/mesa/-/commit/819cbf329a56f1e72a4192b727c7a6d44ad2c2d7