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 <[email protected]>

crates/doc/src/eval.rs

@@ -3638,7 +3638,8 @@ fn eval_feature(doc: &Document, vals: &BTreeMap<String, Value>, f: &Feature, st:
         | FeatureKind::ReverseNormal { .. }
         | FeatureKind::SurfaceRevolve { .. }
         | FeatureKind::SurfaceSweep { .. }
-        | FeatureKind::SurfaceLoft { .. } => surface::eval(doc, vals, f, st),
+        | FeatureKind::SurfaceLoft { .. }
+        | FeatureKind::SurfaceOffset { .. } => surface::eval(doc, vals, f, st),
         FeatureKind::Emboss { .. }
         | FeatureKind::Rib { .. }
         | FeatureKind::ReplaceFace { .. }