Offset Surface: faces copied out at a distance, exact or fitted (#758) (#872)

Offsets picked faces along their normals into a new surface body: planes and cylinders move exactly, other faces get a fitted B-spline within tolerance, and offsets that would fold over are refused. Adds the SurfaceOffset feature (noted under format 18), the surface.offset engine command and an Offset Surface dialog.

Closes #758.

Co-authored-by: jyviko <2501648+jyviko@users.noreply.github.com>

crates/ui-egui/src/timeline.rs

@@ -17,6 +17,7 @@ pub(crate) fn icon_of(k: &FeatureKind) -> &'static str {
         FeatureKind::SurfaceRevolve { .. } => "surface_revolve",
         FeatureKind::SurfaceSweep { .. } => "surface_sweep",
         FeatureKind::SurfaceLoft { .. } => "surface_loft",
+        FeatureKind::SurfaceOffset { .. } => "surface_offset",
         FeatureKind::Revolve { .. } => "revolve",
         FeatureKind::Fillet { .. } => "fillet",
         FeatureKind::Chamfer { .. } => "chamfer",