TSL: Introduce Override Context

[RenaudRohlinger]

Related issue: #32295 (comment)

Description
Implemented a generic overrideContext mechanism in TSL. This allows replacing specific nodes within a context, similar to
replaceDefaultUV but applicable to any node.

Usage example:

const visualizeNormal = Fn( () => {
    return normalLocal.normalize().mul( 0.5 ).add( 0.5 );
} );
// Override normalLocal with a custom vector
const customNormal = vec3( 0, 1, 0 );
const material = new MeshBasicNodeMaterial();
material.colorNode = overrideContext( visualizeNormal(), normalLocal, customNormal );

Added webgpu_tsl_override_context demonstrating normalLocal override on instanced meshes, where normals are transformed (rotated) per instance via the context override.

This contribution is funded by Utsubo & Threejs Blocks

Edit:
Ultimately I want to be able to inline to the stack instead of returning a new node, Almost got it but did hit a quota limit so will wait tomorrow:

material.colorNode = Fn(() => {

	overrideContext( visualizeNormal(), normalLocal, instanceNormal );
	overrideContext( darken(), materialColor, something );

	return materialColor.mul(normalLocal.normalize().mul( 0.5 ).add( 0.5 ))
})()

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	356.97 86.61	356.97 86.61	+0 B +0 B
WebGPU	618.37 173.6	618.6 173.69	+236 B +90 B
WebGPU Nodes	616.97 173.35	617.21 173.44	+236 B +89 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	488.7 121.39	488.7 121.39	+0 B +0 B
WebGPU	690.41 189.47	690.61 189.55	+205 B +82 B
WebGPU Nodes	631.82 172.64	632.02 172.72	+205 B +79 B

[sunag]

I also implemented it, but hadn't committed it: 37daca1
The implementation is quite similar...

Ultimately I want to be able to inline to the stack instead of returning a new node, Almost got it but did hit a quota limit so will wait tomorrow:

I think context substitution should be done by the node flow, or globally via material.contextNode. This way it can conflict with other functions that use this syntax after other function use it, non respecting the flow, only consider the last substitution.

This would be the syntax I was thinking of proposing.

material.contextNode = overrideNode( node, callback );
// or
material.colorNode = someNode.overrideNode( positionLocal, () => positionLocal.mul( .5 ) );

Implemented a generic overrideContext mechanism in TSL. This allows replacing specific nodes within a context, similar to
replaceDefaultUV but applicable to any node.

I have some fears about global replacements; they can be a short-term solution if used as a standard, so I'd like to know more use cases where this is indispensable.