Plane config whose customPlane carries the orthonormal camera basis.
flipped is always false and normal = −viewDir, so the whole model
sits on the KEPT (negative signed-depth) side and every point's VIEW
depth −signedDepth(p, plane) is in (0, viewDepth).
Depth of the kept window along the view direction — the model's full
extent plus padding on both ends. Feed it to SectionConfig's
projectionBelowDepth (with a tiny projectionAboveDepth) so the whole
model projects as solid, and to the hidden-line occluder depth.
A synthetic section plane standing in for the camera.