MeshPhysicalMaterial: Add clearcoatMap and clearcoatRoughnessMap.

[donmccurdy]

Core parts of #18682, without the GLTFLoader changes for now. Note that .x and .y channels are used for the two textures respectively.

[Mugen87]

It would be also good to enhance MaterialLoader and Material.toJSON() so both properties can be serialized/deserialized properly.

three.js/src/loaders/MaterialLoader.js

Lines 259 to 260 in 790811d

	if ( json.clearcoatNormalMap !== undefined ) material.clearcoatNormalMap = getTexture( json.clearcoatNormalMap );
	if ( json.clearcoatNormalScale !== undefined ) material.clearcoatNormalScale = new Vector2().fromArray( json.clearcoatNormalScale );

three.js/src/materials/Material.js

Lines 177 to 185 in 790811d

	if ( this.clearcoat !== undefined ) data.clearcoat = this.clearcoat;
	if ( this.clearcoatRoughness !== undefined ) data.clearcoatRoughness = this.clearcoatRoughness;
	if ( this.clearcoatNormalMap && this.clearcoatNormalMap.isTexture ) {
	data.clearcoatNormalMap = this.clearcoatNormalMap.toJSON( meta ).uuid;
	data.clearcoatNormalScale = this.clearcoatNormalScale.toArray();
	}

[donmccurdy]

Done ✅

[Mugen87]

Reminder: Docs need an update, too. But it's okay to do this in a later PR.

[donmccurdy]

Yep! I'll get the docs later.

[donmccurdy]

It's also lacking an example, but I was just planning to do that with the glTF portion.

[mrdoob]

Thanks!