function GradientManager(zrId, svgRoot) {
    Definable.call(
        this,
        zrId,
        svgRoot,
        ['linearGradient', 'radialGradient'],
        '__gradient_in_use__'
    );
}
Create an SVGElement of displayable and create a <clipPath> of its
clipPath

@param {Displayable} parentEl  parent element
@param {ClipPath[]}  clipPaths clipPaths of parent element
@param {boolean}     isText    if parent element is Text