Improve CairoMakie pdf output

[1-Bart-1]

Currently the 3d plots output by CairoMakie is bad because of the dense meshes.
Flat arrows should be used instead:

"""Flat 2D arrow in 3D: line shaft + triangular mesh head."""
function arrow_flat!(
    ax, origin, direction, tip_width, tip_length,
    plane_normal; color=:black, linewidth=2,
)
    o = collect(Float64, origin)
    d = collect(Float64, direction)
    pn = collect(Float64, plane_normal)
    dir = normalize(d)
    len = norm(d)
    shaft_end = o + (len - tip_length) * dir
    tip_apex = o + len * dir

    perp = cross(dir, pn)
    if norm(perp) < 1e-6
        perp = abs(dir[3]) < 0.9 ?
            cross(dir, [0.0, 0.0, 1.0]) :
            cross(dir, [1.0, 0.0, 0.0])
    end
    perp = normalize(perp) * tip_width

    shaft_vis = o + (len - tip_length * 0.7) * dir
    lines!(ax,
        [Point3f(o...), Point3f(shaft_vis...)],
        color=color, linewidth=linewidth)
    tri = [
        Point3f((shaft_end - perp)...),
        Point3f((shaft_end + perp)...),
        Point3f(tip_apex...),
    ]
    mesh!(ax, tri, [1 2 3], color=color, shading=NoShading)
end