feat: Added wait method on FfmpegChild

[emirror-de]

Every call to spawn without a call on the Result to wait results in a zombie process that stays until the main thread exits. This lead to OS issues with large datasets because it is possible that all PID's are used by the zombies.

This PR exposes the wait method of std::process::Child to the user, giving the possibility to wait for the process to complete and clean up properly.

It has only been tested on Linux.

I am happy to further adjust the code to your coding/documentation style requirements.

Best regards,
Lewin

[nathanbabcock]

Thank you for the PR Lewin 🙏

I tested this branch on Windows and it works for me.

One thing to keep in mind: There is an additional escape hatch on FfmpegChild -- by using the methods as_inner() and as_inner_mut() you can access any method on the inner std::Child instance. Having said that, I like the fact that your change makes the wait method even more easily accessible, and that the comment on spawn() calls attention to a behavior which might otherwise be overlooked. as_inner_mut() can still be used for really uncommon situations like maybe accessing the Child id(), or calling try_wait(), or something else like that.

LGTM! 👍