fix shader tests & cleanup shader code

src/mol-gl/shader/chunks/wboit-write.glsl.ts

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

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

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

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

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