Make sure triangulation returns valid triangles

This check is currently redundant with what the `Triangle` constructor
already does. The constructor will soon get simplified though, and then
the new check will be necessary.

crates/fj-core/src/algorithms/triangulate/delaunay.rs

@@ -53,6 +53,10 @@ pub fn triangulate(
             v2.point_surface,
         ])
         .expect("invalid triangle");
+        assert!(
+            triangle.is_valid(),
+            "Expecting triangles created by triangulation to be valid.",
+        );
 
         let required_winding = match coord_handedness {
             Handedness::LeftHanded => Winding::Cw,