Some changes to use via git subrepo

[fire]

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.

[fire]

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

https://github.com/V-Sekai/V-Sekai.flac/commit/73de59e803ade5cfeb49e52cd33f9ef7c04ca2f0

[fire]

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