Some changes to use via git subrepo

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Hi!

I did some changes for https://github.com/V-Sekai/V-Sekai.flac so that it compiles for Godot Engine 4.2 and through a git subrepo.

https://github.com/V-Sekai/godot/tree/vsk-flac-4.2

Thanks! Feel free to take what you need under your license.

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I removed allocation in the audio thread which is something banned by the audio maintainers and causing github actions test failures with misused memory.

V-Sekai@73de59e

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I wasn't able to correct mono channels in an hour so it's possible to error out if channels != 2.