function Definable(
    zrId,
    svgRoot,
    tagNames,
    markLabel,
    domName
) {
    this._zrId = zrId;
    this._svgRoot = svgRoot;
    this._tagNames = typeof tagNames === 'string' ? [tagNames] : tagNames;
    this._markLabel = markLabel;
    this._domName = domName || '_dom';

    this.nextId = 0;
}
Add a new gradient tag in <defs>

@param   {Gradient} gradient zr gradient instance
@return {SVGLinearGradientElement | SVGRadialGradientElement}
                           created DOM