Return 3D points in Approx

This follows the insights gained in #78. Please check the discussion
there for details.

src/kernel/topology/edges.rs

@@ -74,14 +74,6 @@ impl Edges {
         let mut segments = Vec::new();
         self.approx_segments(tolerance, &mut segments);
 
-        let vertices = vertices
-            .into_iter()
-            .map(|vertex| {
-                // Can't panic, unless the approximation wrongfully generates
-                // points that are not in the surface.
-                surface.point_model_to_surface(vertex).unwrap()
-            })
-            .collect();
         let segments = segments
             .into_iter()
             .map(|Segment3 { a, b }| {
@@ -263,6 +255,6 @@ impl Edge {
 
 /// An approximation of one or more edges
 pub struct Approx {
-    pub vertices: Vec<Point<2>>,
+    pub vertices: Vec<Point<3>>,
     pub segments: Vec<Segment2>,
 }

src/kernel/topology/faces.rs

@@ -109,11 +109,22 @@ impl Face {
         match self {
             Self::Face { edges, surface } => {
                 let approx = edges.approx(tolerance, surface);
-                let mut triangles = triangulate(&approx.vertices);
+
+                let vertices: Vec<_> = approx
+                    .vertices
+                    .into_iter()
+                    .map(|vertex| {
+                        // Can't panic, unless the approximation wrongfully
+                        // generates points that are not in the surface.
+                        surface.point_model_to_surface(vertex).unwrap()
+                    })
+                    .collect();
+
+                let mut triangles = triangulate(&vertices);
                 let face_as_polygon = approx.segments;
 
                 // We're also going to need a point outside of the polygon.
-                let aabb = AABB::from_points(&approx.vertices);
+                let aabb = AABB::from_points(&vertices);
                 let outside = aabb.maxs * 2.;
 
                 triangles.retain(|triangle| {