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/format.rs

@@ -173,6 +173,12 @@
 //!   A `SurfaceRuled` feature (Ruled Surface: `body`, `edges` points, `distance`, optional
 //!   `mode`, `angle`, `direction`, `flip`, `corners` and `face`) is also first saved in 21;
 //!   0.11.0 asks for an update instead of meeting a feature it can't read.
+//!   Under the same version, a `SurfaceExtend` can say how it continues the surface (`mode`:
+//!   `tangent` or `curvature`; `natural` when absent), extend up to something instead of its
+//!   `distance` (`up_to`: a `plane`, a `face` as `body` and `point`, or `bounds`) and keep the
+//!   neighbouring edges as they are (`neighbours: false`). An Extend without them builds the
+//!   planar extension it always did and saves as before; 0.11.0 asks for an update instead of
+//!   dropping them and extending a plane by the distance.
 
 use serde_json::Value;