Don’t plot all celestials all the time

[eggrobin]

Related to #3035.
This uses the hierarchical Keplerian structure of celestials to only plot those that could be visibly separated from their primary at some point along the plotted trajectory.

On a sample 50ish-year trajectory flying by Jupiter and Neptune, looking at the Neptune flyby in Neptune-Sun-Orbit, the framerate goes from 2 to 8 FPS; looking at the Jupiter flyby in Jupiter-Sun-Orbit, from 2 to 4 FPS.

The following table lists the bodies plotted in various situations in RSS, with multi-year trajectories:

Reference frame & camera position	Bodies plotted
ECEF or ECI, near the Earth	Sun, Mercury, Venus, (Earth), Moon, Mars, Phobos, Deimos, Vesta, Ceres, Jupiter, Io, Europa, Ganymede, Callisto, Saturn, Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Iapetus, Uranus, Miranda, Ariel, Umbriel, Titania, Oberon, Neptune, Triton, Pluto, Charon
HCI above the ecliptic, FoV ≈ orbit of Earth	(Sun), Mercury, Venus, Earth, Moon, Mars, Phobos, Deimos, Vesta, Ceres, Jupiter, Io, Europa, Ganymede, Callisto, Saturn, Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Iapetus, Uranus, Miranda, Ariel, Umbriel, Titania, Oberon, Neptune, Triton, Pluto, Charon
HCI above the ecliptic, FoV ≈ orbit of Mars	(Sun), Mercury, Venus, Earth, Moon, Mars, Phobos, Deimos, Vesta, Ceres, Jupiter, Io, Europa, Ganymede, Callisto, Saturn, Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Iapetus, Uranus, Miranda, Ariel, Umbriel, Titania, Oberon, Neptune, Triton, Pluto, Charon
HCI above the ecliptic, FoV ≈ orbit of Jupiter	(Sun), Mercury, Venus, Earth, Moon, Mars, Phobos, Deimos, Vesta, Ceres, Jupiter, Io, Europa, Ganymede, Callisto, Saturn, Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Iapetus, Uranus, Miranda, Ariel, Umbriel, Titania, Oberon, Neptune, Triton, Pluto, Charon
HCI above the ecliptic, FoV ≈ orbit of Saturn	(Sun), Mercury, Venus, Earth, Moon, Mars, Phobos, Deimos, Vesta, Ceres, Jupiter, Io, Europa, Ganymede, Callisto, Saturn, Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Iapetus, Uranus, Miranda, Ariel, Umbriel, Titania, Oberon, Neptune, Triton, Pluto, Charon
HCI above the ecliptic, FoV ≈ orbit of Uranus	(Sun), Mercury, Venus, Earth, Moon, Mars, Phobos, Deimos, Vesta, Ceres, Jupiter, Io, Europa, Ganymede, Callisto, Saturn, Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Iapetus, Uranus, Miranda, Ariel, Umbriel, Titania, Oberon, Neptune, Triton, Pluto, Charon
Neptune-Sun-Orbit, near Neptune	Sun, Mercury, Venus, Earth, Moon, Mars, Phobos, Deimos, Vesta, Ceres, Jupiter, Io, Europa, Ganymede, Callisto, Saturn, Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Iapetus, Uranus, Miranda, Ariel, Umbriel, Titania, Oberon, (Neptune), Triton, Pluto, Charon

[pleroy]

Why not do that all the time? It's not overly costly and it would make the code a bit simpler.

[eggrobin]

That sounds like trading a well-predicted branch for a poorly-predicted one; if we weren’t trying to optimize the square roots by working with squared distances, we would need a null check anyway.