AfterImageNode: Refactor code and improve demo.

[Mugen87]

Related issue: -

Description

The PR slightly refactors the code of AfterImageNode and improves documentation and also makes webgpu_postprocessing_afterimage more interesting. The demo shows how you can use AfterImageNode for implementing a basic "trails" effect for particles.

https://rawcdn.githack.com/mrdoob/three.js/0f4c50f7e0f1008470bed836b398229161d794cf/examples/webgpu_postprocessing_afterimage.html

[Mugen87]

@sunag I had to set transparent to true since the default value of SpriteNodeMaterial is false.

However, SpriteMaterial.transparent is true by default. Is the new default value of SpriteNodeMaterial as expected?

If not, it seems the copy of default values in NodeMaterial.setDefaultValues() does not work in all cases.

three.js/src/materials/nodes/NodeMaterial.js

Lines 1028 to 1060 in 88062ed

	setDefaultValues( material ) {
	// This approach is to reuse the native refreshUniforms*
	// and turn available the use of features like transmission and environment in core
	for ( const property in material ) {
	const value = material[ property ];
	if ( this[ property ] === undefined ) {
	this[ property ] = value;
	if ( value && value.clone ) this[ property ] = value.clone();
	}
	}
	const descriptors = Object.getOwnPropertyDescriptors( material.constructor.prototype );
	for ( const key in descriptors ) {
	if ( Object.getOwnPropertyDescriptor( this.constructor.prototype, key ) === undefined &&
	descriptors[ key ].get !== undefined ) {
	Object.defineProperty( this.constructor.prototype, key, descriptors[ key ] );
	}
	}
	}

[sunag]

Yes, sprites should be transparent by default.

If not, it seems the copy of default values in NodeMaterial.setDefaultValues() does not work in all cases.

It would just define values ​​that do not exist in the class in order to save some computation when cloning the classes, of course this could be solved by testing, but I think this approach is a hack and should be discontinued at some point and replaced by something like #28328