utils: implement AsAny only on OpaqueObject

[kchibisov]

Given that AsAny was implemented for pretty much anything, it was really easy to use it on objects that were not really implementing AsAny.

[kchibisov]

That ensures that the we cast to the right thing, since in some cases it was able to cast to wrapping container instead of the type that is wrapped. It's a bit annoying, but should help with random bugs.

[madsmtm]

Hmm, I don't really see the value of this, so will neither "approve" nor "request changes" (if you're really blocked by this, then I'm fine with merging it).

The plan is to get rid of the platform modules anyhow, right? When would you hit this otherwise? In user code? If so, then I'd rather expose something like the following:

impl dyn Window {
    pub fn as_inner<T: 'static + Window>(&self) -> Option<&T> {
        let this: &(dyn std::any::Any + '_) = self.as_any();
        this.downcast_ref::<T>()
    }
}

// Similarly for ActiveEventLoop, MonitorHandle, etc.

This is also a bit cleaner than the self.as_any().downcast_ref::<crate::platform_impl::Window>().unwrap() that we do everywhere now, would simplify to self.as_inner::<crate::platform_impl::Window>().unwrap().

(I consider AsAny a hack anyhow, and one that we'll be able to remove once we have MSRV 1.86, since by then rust-lang/rust#65991 will have landed).

[kchibisov]

The problem here is that T: Any implements AsAny for e.g. Arc, and when you call the method as_any on that Arc the cast will fail, since you'd need a manual deref() because the Arc had as_any implemented for it, because it implements Any. By using OpaqueObject we avoid that issue and when calling as_any on Arc we call the as_any on the T inside the Arc, thus lowering the odds of casting incorrectly, because of nested containers.

It's mostly to not make a mistake in internal code, since AsAny was implemented for all rust types.

[madsmtm]

I've opened #4160, I prefer that alternative.