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_geometry.rs

@@ -139,11 +139,21 @@ macro_rules! geometry_walk {
                         f(Pos(start));
                     }
                 }
-                FeatureKind::Chamfer { edges, .. } | FeatureKind::SheetHem { edges, .. } | FeatureKind::SurfaceExtend { edges, .. } => {
+                FeatureKind::Chamfer { edges, .. } | FeatureKind::SheetHem { edges, .. } => {
                     for e in edges {
                         f(Pos(e));
                     }
                 }
+                FeatureKind::SurfaceExtend { edges, up_to, .. } => {
+                    for e in edges {
+                        f(Pos(e));
+                    }
+                    match up_to {
+                        Some(crate::ExtendTo::Plane(plane)) => f(Plane(plane)),
+                        Some(crate::ExtendTo::Face(t)) => f(Pos(& $($m)? t.point)),
+                        Some(crate::ExtendTo::Bounds) | None => {}
+                    }
+                }
                 FeatureKind::Shell { faces, .. }
                 | FeatureKind::OffsetFace { faces, .. }
                 | FeatureKind::DeleteFace { faces, .. }