TSL: Introduce varying.setInterpolation()

[cmhhelgeson]

Related issue: #30576

Description

Modifies VaryingNode, allowing the user to directly define interpolation and sampling types for a varying node. In cases where no interpolation is defined, the default optimizations/behavior stays in place.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	336.39 78.35	336.44 78.37	+52 B +16 B
WebGPU	540.78 149.8	541.67 150.04	+893 B +236 B
WebGPU Nodes	540.25 149.7	541.14 149.93	+893 B +236 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	465.37 112.21	465.37 112.21	+0 B +0 B
WebGPU	613.67 165.87	614.51 166.09	+841 B +219 B
WebGPU Nodes	568.66 155.28	569.5 155.49	+841 B +210 B

[cmhhelgeson]

I decided to just use @isaac-mason's basic sample for now, leaving the creation of a more dynamic sample for a later time. If possible, please include Isaac as a contributor to this PR.

[sunag]

It would be interesting to have the variables in the constants. For example: THREE.InterpolationType.PERSPECTIVE, THREE.InterpolationSampling.CENTROID...

[Mugen87]

@cmhhelgeson It seems the options flat first and flat either in webgpu_centroid_sampling produce shader errors:

Error while parsing WGSL: :27:37 error: expected interpolation sampling name
Did you mean 'first'?
Possible values: 'center', 'centroid', 'either', 'first', 'sample'
@location( 3 ) @interpolate( flat, flat first ) testUV : vec2,

Is that a regression?

[cmhhelgeson]

Collaborator

No this behavior is incorrect. The @interpolate tag should only be (flat, first), not (flat, flat first). This would be an error in how the code is being constructed.

[cmhhelgeson]

I think this might actually just be an issue with the sample, I'll look into it.