add occlussion color

CHANGELOG.md

src/apps/docking-viewer/viewport.tsx

src/examples/lighting/index.ts

src/extensions/cellpack/model.ts

src/mol-canvas3d/passes/postprocessing.ts

src/mol-gl/shader/postprocessing.frag.ts

src/mol-plugin-ui/structure/quick-styles.tsx

src/mol-plugin/util/headless-screenshot.ts