Extend any surface: natural, tangent and curvature, up to a target (#760) (#1166)

Extend works on any surface: natural (continuing the face's own surface), tangent (G1 run-off) or curvature (G2 run-off), by a distance, up to a plane or face, or back to the surface's bounds, with neighbouring edges lengthened to meet. Untrim and Extend share face parameter-loop helpers. The new SurfaceExtend fields are noted under format 21. Closes #760.

crates/doc/src/feature_refs.rs

@@ -175,8 +175,15 @@ macro_rules! feature_walk {
                     }
                 }
                 FeatureKind::SurfaceUntrim { face, .. } => f(B(& $($m)? face.body)),
+                FeatureKind::SurfaceExtend { body, up_to, .. } => {
+                    f(B(body));
+                    match up_to {
+                        Some(crate::ExtendTo::Plane(plane)) => $plane(plane, f),
+                        Some(crate::ExtendTo::Face(t)) => f(B(& $($m)? t.body)),
+                        Some(crate::ExtendTo::Bounds) | None => {}
+                    }
+                }
                 FeatureKind::SheetConvert { body, .. }
-                | FeatureKind::SurfaceExtend { body, .. }
                 | FeatureKind::SurfaceRuled { body, .. }
                 | FeatureKind::Unstitch { body, .. }
                 | FeatureKind::ReverseNormal { body, .. }