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Additionally, this resolves the remaining GPLv2 only licensed files by relicensing them to GPLv2.0-or-later.
CallbackStatus instances aren't the cheapest things to copy around
(relative to everything else), given that they're currently 520 bytes in
size and are currently copied numerous times when callbacks are invoked.
Instead, we can pass the status by const reference to avoid all the
copying.
Liam