AppKit: Fix window content scaling like a static image on programmatic maximize/restore

[yay]

This fixes a macOS programmatic maximize/restore case that shows up with custom titlebars.

When a resizable AppKit window is maximized through Window::set_maximized, winit calls NSWindow::zoom. AppKit runs that zoom animation synchronously and emits intermediate live-resize callbacks while the animation is in progress.

If set_maximized is requested from inside a winit event callback, winit is already handling an event when NSWindow::zoom starts. In that state, the intermediate resize/redraw work cannot be delivered to the application until after the zoom call returns. The visible result is that the window animation stretches the previous layer contents like a static image instead of redrawing the app at the intermediate window sizes.

This PR defers only the reentrant macOS set_maximized path to the next main run-loop turn. That lets AppKit run the zoom animation after the current winit event callback returns, so intermediate resize/display callbacks can be processed normally during the animation.

This is related to but separate from #4588. That PR improves AppKit live-resize redraw timing once AppKit is driving resize callbacks. This PR avoids starting AppKit's zoom animation while winit is already inside an application event callback.

I used a small eframe/wgpu demo with a custom toolbar to reproduce this: double-clicking the toolbar background sends ViewportCommand::Maximized(...). Before this fix, maximize/restore visually stretches the old window contents during the native animation. After this fix, the app relayouts/redraws during the animation.

Before:

winit-zoom-before.mp4

After:

winit-zoom-after.mp4

[kchibisov]

Wouldn't it be an issue for fullscreen,etc? Basically state changes that trigger animation? If so would be nice to have similar logic in them as well?

[yay]

@kchibisov Good point. I generalized the deferral into defer_if_handling_event and now use it for both set_maximized and fullscreen transitions.