wip, solid impostor flag

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

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

src/mol-gl/renderable/cylinders.ts

src/mol-gl/renderable/spheres.ts

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

src/mol-gl/shader/cylinders.vert.ts

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