loadTexture( textureIndex ) {

    		const json = this.json;
    		const options = this.options;
    		const textureDef = json.textures[ textureIndex ];
    		const source = json.images[ textureDef.source ];

    		let loader = this.textureLoader;

    		if ( source.uri ) {

    			const handler = options.manager.getHandler( source.uri );
    			if ( handler !== null ) loader = handler;

    		}

    		return this.loadTextureImage( textureIndex, source, loader );

    	}
Specification: https://github.com/KhronosGroup/glTF/tree/master/specification/2.0#textures
@param {number} textureIndex
@return {Promise<THREE.Texture>}