Support (combining) multi-track (glTF) animations

[appgurueu]

The purpose of this is twofold:

The advanced benefit is that it closes #12663, letting modders combine multiple animation tracks (e.g. in player_api, no separate "walk + dig" animation would be needed: the two animations can simply be combined). This is very useful if the cartesian products of possible combinations would otherwise get large.

The other is more of a convenience that it lets you address named animations as found in glTF files. This makes it easier to just take such files off-the-shelf, no need to bake everything into a single track and be careful with picking the right frame numbers.

To do

This PR is Ready for Review.

I would love to have some feedback by mod / game devs on the API design.

There are two limitations: I did not bother with extending this feature to local animations or support for the model[] formspec element. Both can come later, though I'm not sure whether there's much point in supporting the former at all. There is also a non-negligible chance that we'll be able to make SSCSM APIs that obsolete both features happen.

_{(Minor TODO: Get permission to use the test model or replace it)}

How to test

• There should be no regressions related to skeletal animation. All of the following should still work:
  • Animated .x, .b3d, and .gltf / .glb models (e.g. those in devtest);
  • Animation blending;
  • Bone overrides including interpolation;
  • Local player animations;
  • model[] formspec element
• The new feature should work. It can be tested rudimentarily using the gltf:multi_track entity: Punch to toggle one animation, right-click to toggle the other.

[appgurueu]

Rebased.

[Wbjitscool]

would it be possible for player eating animations too in the future?

[appgurueu]

would it be possible for player eating animations too in the future?

I'm not sure what you're asking, maybe come again?

This is a feature that lets you simultaneously play multiple animation tracks for gltf models.

I imagine this could be used for eating animations, e.g. with first-person attachments and by making use of the observers API.

[kromka-chleba]

I took a brief look at the API and it looks decent.
I'm going to test this later.

(Minor TODO: Get permission to use the test model or replace it)

I can make a replacement model under CC BY-SA or CC0