impostor per-pixel object clipping

CHANGELOG.md

src/mol-gl/shader/chunks/clip-instance.glsl.ts

src/mol-gl/shader/chunks/clip-pixel.glsl.ts

src/mol-gl/shader/chunks/common-clip.glsl.ts

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

src/mol-gl/shader/direct-volume.frag.ts

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