Is your feature request related to a problem? Please describe.
Varying opacity can be useful to visualize overlapping or occluded data.

Seems like most primitives support RGBA arguments, however the alpha channel was ignored for the one I tested (rr.log_line_segments) or didn't correspond to opacity (rr.log_mesh).

Describe the solution you'd like
Would be great if all primitives (including 3D primitives) support some form of transparency / alpha compositing.