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/params.rs

@@ -354,7 +354,7 @@ macro_rules! feature_walk {
                 FeatureKind::Thicken { thickness, .. } => f("Thickness", thickness, L, I),
                 FeatureKind::SurfaceTrim { plane, .. } => $plane(plane, f),
                 FeatureKind::SurfaceExtend { distance, .. } => f("Distance", distance, L, I),
-                FeatureKind::Unstitch { .. } | FeatureKind::ReverseNormal { .. } => {}
+                FeatureKind::Unstitch { .. } | FeatureKind::ReverseNormal { .. } | FeatureKind::SurfaceUntrim { .. } => {}
                 FeatureKind::SurfaceOffset { distance, tolerance, .. } => {
                     f("Distance", distance, L, I);
                     if let Some(t) = tolerance {