wip, link repr, dashed cylinders

...presentation/structure/visual/inter-unit-link-cylinder.ts

...presentation/structure/visual/intra-unit-link-cylinder.ts

src/mol-geo/representation/structure/visual/util/link.ts

src/mol-geo/shape/mesh-builder.ts

src/mol-util/bit-flags.ts

src/mol-view/stage.ts