C++: Expose raw window handle API

[redstrate]

We had the raw_window_handle_06 feature and functions in Rust, but there wasn't an easy way to do the same in C++. This is especially useful for embedding windows (such as browsers, 3D views, etc.) or just wanting to access the platform's handles for other reasons.

Similar to Rust, you need to enable a specific non-default feature for this API. I re-used the existing NativeWindowHandle struct for this, moving it out of slint-platform.h and adding getters to it.

Examples

void print_native_handles(slint::Window& window)
{
	auto native_handle = window.native_window_handle();

	// Wayland
	std::cout << "- wl_display = " << native_handle.wayland_display() << std::endl;
	std::cout << "- wl_surface = " << native_handle.wayland_surface() << std::endl;

	// Win32
	std::cout << "- hwnd = " << native_handle.win32_hwnd() << std::endl;
	std::cout << "- hinstance = " << native_handle.win32_instance() << std::endl;
}

TODO

• See if there's an existing issue (currently I only came across C++ Raw Window Handle #6248)
• Double-check if this is an API break, I don't think so since slint-window.h/slint-platform.h should be interchangeable.
• Test on macOS/Windows
• Add/expand the documentation
• Do we need a way to check the underlying window handle type? But this can probably be answered later.
• Outside of this PR, some improvements that can be upstreamed now: Maybe moving NativeWindowHandle, disabling Win32 properly on Linux

Notes

I intentionally only added getters for Wayland/Appkit/Win32 since that's the only platforms that currently matter to me. Since we didn't have a C++ API at all before that's still a net positive, and this can be easily extended later.

It should be safe to call this even if the window isn't initialized, or on the wrong platform.

[ogoffart]

Thanks for the contribution. I think it indeed make sense to expose that, and it could be exposed to C++ without a feature.

[ogoffart]

The reason it is a feature in Rust is because it is dependent of a specific Rust crate (raw-window-handle) and a specific version (0.6) and that in the future, Rust users may opt for a newer version of raw-window-handle and so wouldn't enable this feature anymore.

In C++ we don't have this as we'd have to re-expose the API anyway, so IMHO it shouldn't need to be a feature and it can be enabled by default

[ogoffart]

Don't we also need a getter for the NSWindow?

[ogoffart]

(maybe put things in a module to avoid repeating this too often?)

[ogoffart]

This could get safety comment. There was a comment before

[ogoffart]

Actually, looking again at my comment in #6248 (comment) , I wonder if we really need this NativeWindowHandle indirection and can just have things like win32_handle() directly on the Window.

@tronical: opinions?

[tronical]

I think you're right. That would make for a simpler API. Just one function per platform (two for linux).