single and multi layer screen-door transparency

src/mol-geo/geometry/transparency-data.ts

src/mol-gl/renderable/direct-volume.ts

src/mol-gl/renderable/schema.ts

src/mol-gl/shader/chunks/assign-material-color.glsl

src/mol-plugin/state/transforms/helpers.ts

src/mol-plugin/state/transforms/representation.ts

src/mol-repr/visual.ts

src/mol-theme/transparency.ts