Add TSL easing functions based on tsl-easings

[devallibus]

Summary

Add Robert Penner's easing functions as a TSL utility shader, sourced from @y11i-3d/tsl-easings by Yuichiroh Arai.

Upstream

• Repository: https://github.com/y11i-3d/tsl-easings
• npm: @y11i-3d/tsl-easings v1.0.0
• License: MIT (2026 Yuichiroh Arai for TSL port; 2001 Robert Penner for original equations)
• Demo: https://y11i-3d.github.io/tsl-easings/
• Peer dep: Three.js >=0.183.0

Available Functions (30 total)

10 families × 3 variants (In, Out, InOut):

Family	In	Out	InOut
Sine	sineIn	sineOut	sineInOut
Quad	quadIn	quadOut	quadInOut
Cubic	cubicIn	cubicOut	cubicInOut
Quart	quartIn	quartOut	quartInOut
Quint	quintIn	quintOut	quintInOut
Expo	expoIn	expoOut	expoInOut
Circ	circIn	circOut	circInOut
Back	backIn	backOut	backInOut
Elastic	elasticIn	elasticOut	elasticInOut
Bounce	bounceIn	bounceOut	bounceInOut

Back and Elastic also have custom* variants accepting parameters (overshoot, amplitude/period).

All functions accept float, vec2, vec3, or vec4 TSL nodes.

What to do

1. Create a tsl-easings shader entry in shaders/tsl-easings/ with:

   • shader.json manifest with language: "tsl", proper provenance pointing to upstream repo, Robert Penner attribution, and Yuichiroh Arai credit
   • source.ts containing the easing functions (adapted to ShaderBase's createMaterial contract or as a utility export)
   • Recipe showing usage in a Three.js material

2. Provenance: sourceKind: "port" with sources linking to the upstream repo and Robert Penner's original equations

3. Compatibility: Note the higher Three.js minimum (>=0.183.0) vs our existing TSL shader (>=0.170.0)

4. Consider: Whether to ship as one combined tsl-easings shader or split per family (e.g. tsl-ease-sine, tsl-ease-bounce). A single entry is simpler and matches how the upstream ships it.

Credit

• Original easing equations: Robert Penner (BSD license, 2001)
• TSL implementation: Yuichiroh Arai (@y11i-3d) (MIT, 2026)