Fix animations and attachments once and for all

[appgurueu]

This is supposed to fix skeletal animation / attachment related jank, specifically #15186, #14818, #14817 and to refactor a part of the code in preparation of e.g. #9218. See also #15350.

It works by doing things the way they should be done. Currently we have Irrlicht doing things wrong and 2-3 layers of workarounds, some in Irrlicht, some in Luanti, on top of it. This is a broken mess.

What should be done is conceptually pretty simple: First, we need to recursively update the absolute transforms of node hierarchies consisting of scene nodes (including bones) inside OnAnimate.
When rendering, we simply transfer these transforms to the mesh and let it do the CPU skinning. (Later we will want to do that on the GPU.)
Each animated scene node must store its own bone transformations for animation blending purposes (and later GPU skinning).

All the workarounds we currently have can essentially go out the window.

To do

This PR is WIP. I'm currently aiming for 5.11.0 since this will need a decent amount of testing.

• Figure out why the heck .x has just ceased working it has unceased working
• Replace hints with binary search, refactor this to use a sort of "keyframes" struct (I have this around somewhere) Should be all nice and tidy now :)
• Fix stale nametag positions (figure out where to best update their position) can't reproduce (anymore?), they should be updated at the right point in time anyways (after rendering of everything 3d and thus after all absolute transforms have been computed)
• Pull out debug remnants
• Check whether build times are still comparable
• Do proper bounding box calculations
• Do bone overrides properly

How to test

See the related issues.

[hecktest]

For what it's worth, the hatlag is gone again but all root entities now jitter around a lot, all the time. As if interpolation wasn't working properly.
Other than that it looks like everything is on the right track.

[appgurueu]

I have started to cleanly split this up into the "refactor" and "fix" parts, otherwise it's unmanageable for me to get to a reviewable state.

As if interpolation wasn't working properly.

Indeed it isn't. Maybe you have an idea what's going wrong? Here's the PR with just the refactoring changes in relatively clean commits: #15522. Nevermind, it was just a dumb argument swap due to inconsistent Irrlicht conventions.

[appgurueu]

Closing in favor of #15722.