wip, point rendering

src/apps/canvas/component/structure-representation.tsx

src/mol-geo/geometry/point/point.ts

src/mol-geo/representation/structure/units-visual.ts

src/mol-gl/_spec/renderer.spec.ts

src/mol-gl/renderable.ts

src/mol-gl/renderable/mesh.ts

src/mol-gl/renderable/point.ts

src/mol-gl/scene.ts

src/mol-gl/shader/point.frag