added global picking threshold for transparent objects

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

src/mol-gl/renderable.ts

src/mol-gl/renderable/direct-volume.ts

src/mol-gl/renderable/lines.ts

src/mol-gl/renderable/mesh.ts

src/mol-gl/renderable/points.ts

src/mol-gl/renderable/schema.ts

src/mol-gl/renderer.ts

src/mol-gl/shader/chunks/assign-material-color.glsl

src/mol-gl/shader/chunks/common-frag-params.glsl

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

src/mol-gl/shader/lines.frag

src/mol-gl/shader/mesh.frag

src/mol-gl/shader/points.frag