Fix moving-object detection for nested AnimationGroups with z-indexed mobjects

[Merzlikin-Matvey]

Overview: What does this pull request change?

Fixes an AnimationGroup behaviour with negative z_index Mobjects. More about this problem you can see in these issues:
#3334 and #3914

Motivation and Explanation: Why and how do your changes improve the library?

Mobjects with negative z_index have problems with playing in animations. This PR fixes it

Links to added or changed documentation pages

There are no changes to the documentation

Further Information and Comments

I had already opened a pull request, but I decided to close it due to many issues with the previous solution. I have taken all the shortcomings, now there are no performance or graphical test issues.

Related issues

Closes #3334
Closes #3914

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested

[behackl]

Thanks! Please take a look at my suggestion.

[behackl]

Thanks again!
I took a closer look at the earlier suggested fix, and it turns out that correctly handling nested AnimationGroups requires recursively traversing subanimations. For simplicity, that recursive helper is now implemented locally inside the moving-object detection method (get_moving_mobjects) rather than as a separate scene method.

I also ran the relevant negative z-index graphical tests locally, and they pass. LGTM.