WebGPURenderer: Fix receiveShadow and introduce shadowPositionNode

[RenaudRohlinger]

Description

Currently, setting material.receiveShadow = false does not affect the shadow behavior. This pull request fixes the issue.
This pull request also introduces shadowPositionNode which allows the modification of the comparison point for objects receiving the shadow.

There is a current limitation with light.shadowNode, which globally affects all the meshes receiving a shadow from a light, and material.shadowNode, which modifies the behavior of the mesh that casts a shadow. material.shadowPositionNode modifies the behavior of the mesh that receives the shadow.

This allows the alignment of shadows on custom surfaces modified by nodes or even custom effects on specific surface, for example:

planeMaterial.shadowPositionNode = tslFn( () => {

	const pos = positionWorld.toVar();
	pos.xz.addAssign( mx_fractal_noise_vec3( positionWorld.mul( 2 ) ).saturate().xz );
	return pos;

} )();

PS: Since it's more about the pixels rather than the position of the vertices, would something like shadowCompareNode be better? /cc @sunag

This contribution is funded by Utsubo

[sunag]

Very nice! Do you think about adding the example updated?

[RenaudRohlinger]

Good idea! Done 👍

[sunag]

Can we create an offset of the normals instead of changing the position? We could use it on other types of surfaces, this seems limiting to me since normalWorld continues with the reference from the previous position.

An update I haven't made yet... I think we should test is that the leftover normals should follow the object's normalMap changes.