Platform-specific getters on WindowAttributes

[daxpedda]

Currently WindowAttributess purpose is to expose the current state of WindowBuilder. But it doesn't support platform-specific fields.
It would be nice to move PlatformSpecificWindowBuilderAttributes into WindowAttributes so we can do that.

Alternatively, I'm not entirely sure why WindowAttributes exists in the first place if seemingly we could let users just get free (mutable) access to WindowBuilder fields and the ones we don't want them to get access to can have getters/setters.

What do you guys think?

[notgull]

Personally I think it's a good value add over having the structure's fields be exposed normally (semver issues). Although I support having the ability to access those platform specific fields as well. It would make it easier to make fuller wrappers around winit (e.g. async-winit)

[madsmtm]

I dislike WindowAttributes, and think it should've been private to begin with; it allows us to more easily change implementation details, while still exposing a stable API behind the setter methods. But then again, Winit is perma-unstable, so eh, maybe it doesn't really matter.

Back when I tried to remove it in #2134, it was met by resistance because there were no getters, and that broke people relying on being able to access the current builder values.

[daxpedda]

See #2613 for the discussion on why it was reverted.

Generally speaking I really do dislike WindowAttributes, but I've learned over time that having builders without getters will always spawn requests for them, so I tend to consider them "good API design".

Personally I think we either remove WindowAttributes and introduce a lot of getters, or leave WindowAttributes in place.

In the meantime I will go ahead and move PlatformSpecificWindowBuilderAttributes into WindowAttributes.