more granular control over obstacle visibility

would be nice — atm the option is hide all or show all
for 2D one could currently use color vectors with 0 alpha for invisible obstacles, but in 3D its more difficult

pseudo code idea would be:

on(obstacleplot.visible) do v
    if v is a Scalar
        mesh.visible = v
        mesh[1] = _get_meshes.(obstacleplot[1])
    else
        mesh.visible = any(v)
        mesh[1] = [_get_meshes(o) for (i,o) in enumerate(obstacleplot) if v[i]]
    end
end