_getOutlineMaterial( originalMaterial ) {

		let outlineMaterial = this._materialCache.get( originalMaterial );

		if ( outlineMaterial === undefined ) {

			outlineMaterial = this._createMaterial();

			this._materialCache.set( originalMaterial, outlineMaterial );

		}

		return outlineMaterial;

	}
For the given toon material, this method returns a corresponding
outline material.

@private
@param {(MeshToonMaterial|MeshToonNodeMaterial)} originalMaterial - The toon material.
@return {NodeMaterial} The outline material.