While it’s unlikely, it’s possible that different MoltenVK versions
could require their own compatability patches. Support that by making
the `moltenvk` derivation provide the patch via `passthru`. There is no
package with the patch applied because the patch should never be used by
anything other than DXVK.
- Move the synchronization primitive changes to their own patch, so it
can be applied conditionally on Darwin. Also, document why this change
is needed; and
- Refactor how `src` is handled to support allowing Darwin and Linux to
diverge in case the Darwin patches do not apply to the latest version.
This should address some post-merger concerns that were raised about
Darwin’s blocking updates for Linux.
