message: chore: add PostProcessStage stories
diff: import React from "react";
  import { storiesOf } from "@storybook/react";
  +import { Cartesian3 } from "cesium";
  import Viewer from "../Viewer";
  -import PostProcessStage from "../PostProcessStage";
  +import Entity from "../Entity";
  +import * as postProcessStage from "../PostProcessStage";
  +import * as postProcessStageComposite from "../PostProcessStageComposite";
  +import model from "./assets/Cesium_Air.glb";
  const shader = `
  uniform sampler2D colorTexture;
  @@ -26,9 +30,30 @@ void main(void)
  `;
  export default () => {
  - storiesOf("PostProcessStage", module).add("default", () => (
  + storiesOf("PostProcessStage", module)
  + .add("Mosaic", () => (
  <Viewer full>
  - <PostProcessStage fragmentShader={shader} />
  + <postProcessStage.PostProcessStage fragmentShader={shader} />
  + </Viewer>
  + ))
  + .add("Lens flare", () => (
  + <Viewer full>
  + <postProcessStage.LensFlareStage intensity={5} />
  + </Viewer>
  + ))
  + .add("Night vison", () => (
  + <Viewer full>
  + <postProcessStage.NightVisionStage />
  + </Viewer>
  + ))
  + .add("Bloom", () => (
  + <Viewer full>
  + <postProcessStageComposite.Bloom />
  + <Entity
  + position={Cartesian3.fromDegrees(-74.0707383, 40.7117244, 100)}
  + model={{ uri: model }}
  + tracked
  + />
  </Viewer>
  ));
  };
  