Fix onion skinning internals activating audio/method/animation tracks

[RandomShaper]

This should fix #83426, but I'd need it to be tested. @VansonLeung, would you give it a try, please?

[VansonLeung]

EDIT: re-test success with the proper practice. Please ignore my mini-project scenario.

---

My re-test failed:

retry.mp4

Mini - project:
gd_anim_unittest_1 2.zip

OS:
Mac OS 13.4, arm64

[VansonLeung]

FYI - Here is my test on Godot 4.1 stable version:

By considering this - I think the real trigger cause of this issue may not originate from onion skinning, but instead from something else. Turning on Onion Skinning just makes this issue happen to be more noticeable. @RandomShaper

If that is the case - should I also take a look at the part of the codes where audio animation key is being triggered? Or - is it an expected behaviour for Godot 4.1?

If the issue turns out to be recognized as a bug originating from the other issue stated in the link below, this particular pull request of yours may be optional / omitted / may need further fixes.

Edit: added a new issue: #83436

oldretry.mp4

[VansonLeung]

Per this it seemed that it was me doing the audio track animation key wrongly.

Ref: #83436

So I could say I tested your commit successfully with the stated issue #83426 fixed now. (Albeit the bug being practically not reproducible anymore when following the correct practice, even without this commit fix.)

[akien-mga]

Thanks!