ReadonlygroupAdd it to the THREE.Scene, untransformed (results are in world
coordinates; a transformed ancestor is detected and warned about once).
Per bound mesh the section cuts, it holds a LineSegments of the contours
and a Mesh of the fill (empty while the section has no fill).
ReadonlyplaneThe section plane, kept equal to the scene's. Three clips away what lies on the plane's negative side, so the part on the side the section normal points to stays visible.
A section drawn by bindSection.