Gradient = function (colorStops) {

    this.colorStops = colorStops || [];

}
Displayable for incremental rendering. It will be rendered in a separate layer
IncrementalDisplay have two main methods. `clearDisplayables` and `addDisplayables`
addDisplayables will render the added displayables incremetally.

It use a not clearFlag to tell the painter don't clear the layer if it's the first element.