The model's centre projected onto the plane: where a handle sits.
A square on the plane the clip uses (centred on anchor, sized from the model bounds), in winding
order. It lies in the clipping plane exactly; the drawn preview quad of a rotated cardinal plane
is axis-aligned instead, so place marks from anchor and the plane, not by matching that quad.
The plane's id, when it has one (the primary plane's too).
Unit normal, and dot(p, normal) = distance on the plane.
True for RenderOptions.sectionPlane; false for an entry of extraSectionPlanes.
One section plane in world space.