Custom bones in armor don't respect custom bone parents' transformations

*Partner Rank:*

2

*Partner Team Reporting:*

Everbloom Studios

*Verification builds:*

1.13.0.16

*Summary:*

Custom bones in armor geometry don't respect their custom parent bones' transformations. This means, non-vanilla bones in armor cannot form a hierarchy, as all the custom bones are displayed as if their direct parent was the same. They do, however, respect the transformation of their closest vanilla ancestor.

*Impact:*

We are unable to create complex geometry to override armor, and are limited to non-rotated cubes.

*Repro Steps:*

1. Create and join the provided world.
2. In this world's resource pack, the diamond helmet has been overwritten to add horns to its model.
3. There should be two entities in front of you. On the left is an armor stand wearing a real diamond helmet, and to the right there's a custom entity, which is displaying the player helmet geometry directly as if it were its own model.
4. Compare the two helmets.

*Observed Results:*

The two helmets don't look alike. The real helmet has all the custom cubes displayed at the origin (the neck) of their closest vanilla ancestor (the head), without any rotation.

*Expected Results:*

The two helmets should look identical, with horns on top of a vanilla helmet

*Screenshots/Videos attached: Yes/No*

Yes. The screenshot shows the bugged behavior to the left (on the armor stand), and the expected appearance to the right.

*Regression Builds:*

Unknown

*Notes:*
A player_armor.json has also been attached, which is an edit of the vanilla file with extra rotated bones to form horns, whose closest vanilla parent is 'head'.
This bug is NOT MCM-817, as that bug appeared much more recently than this one. MCM-817 involves a very similar problem with custom armor bones that don't support a hierarchy but, unlike with this bug, do not respect their vanilla ancestors either. MCM-817 and this could very well be related.