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/kernel/src/lib.rs

@@ -78,6 +78,7 @@ mod step;
 mod step_in;
 mod step_out;
 mod surface_kind;
+mod surface_offset;
 mod surfaces;
 mod sweep_path;
 mod tess;
@@ -116,6 +117,7 @@ pub use step::step_export;
 pub use step_in::{ImportedBody, StepImport, StepNode, step_import, step_import_shared, step_orientation_errors, step_validate};
 pub use step_out::{ExportBody, ExportProduct, StepHeader, step_export_bodies, step_export_products};
 pub use surface_kind::{FaceSurface, face_surface};
+pub use surface_offset::offset_surface;
 pub use surfaces::{
     copy_faces, extend, extrude_sheet, loft_sheet, patch_edges, patch_region, remove_faces, reverse_faces, revolve_sheet, stitch, sweep_sheet,
     sweep_sheet_points, thicken, trim, unstitch,