This, strictly speaking, doesn't _need_ to be public domain any more, since
SDL3 no longer inserts this code into apps via a static SDL_main library; it's
just part of the normal zlib-licensed SDL library and called into through a
well-defined entry point at startup, now.
But removing a public domain notice isn't the done thing, so we'll leave it in.
(To a related point: even if others have rewritten good portions of this code,
they're doing it under the assumption that this specific file has a public
domain license.)
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