Three overlapping transparent: true planes slide back and
forth through each other. Their draw order changes every frame -
prepareScene() re-sorts them back to front by view-space
depth, so the blending always composites correctly, no matter which
one is currently nearest. The plane on the right is
side: Side.FRONT and spins around Y: half the turn it is
culled away entirely. Edit its side to
Side.DOUBLE below to see it stop disappearing.