Untrim: holes removed and a face restored to its natural bounds (#1150)

Adds surface.untrim (#761); the SurfaceUntrim feature is first saved in format 21.

crates/doc/src/format.rs

@@ -167,6 +167,9 @@
 //!   Under the same version, looks can explicitly select opaque PBR parameters (`pbr`: metal
 //!   fraction, roughness, dielectric IOR). Absent fields keep the scalar shading a look always
 //!   had; 0.11.0 asks for an update instead of silently dropping the response.
+//!   A `SurfaceUntrim` feature (Untrim: a `face` as `body` and `point`, optional `holes`
+//!   points, `outer` and `in_place`) is also first saved in 21; 0.11.0 asks for an update
+//!   instead of meeting a feature it can't read.
 
 use serde_json::Value;