Add Triangle::is_valid

The method doesn't make sense yet, as `Triangle::new` explicitly forbids
degenerate triangles. This is about to change though, and adding this
method is preparation for that.

crates/fj-math/src/triangle.rs

@@ -1,3 +1,4 @@
+use approx::AbsDiffEq;
 use parry3d_f64::query::{Ray, RayCast as _};
 
 use crate::Vector;
@@ -41,6 +42,26 @@ impl<const D: usize> Triangle<D> {
         self.points
     }
 
+    /// # Determine whether the triangle is valid
+    ///
+    /// A triangle is valid, if it is not degenerate. In a degenerate triangle,
+    /// the three points do not form an actual triangle, but a line or even a
+    /// single point.
+    ///
+    /// ## Implementation Note
+    ///
+    /// Right now, this function computes the area of the triangle, and compares
+    /// it against [`Scalar`]'s default epsilon value. This might not be
+    /// flexible enough for all use cases.
+    ///
+    /// Long-term, it might become necessary to add some way to override the
+    /// epsilon value used within this function.
+    pub fn is_valid(&self) -> bool {
+        let [a, b, c] = self.points;
+        let area = (b - a).outer(&(c - a)).magnitude();
+        area > Scalar::default_epsilon()
+    }
+
     /// Normalize the triangle
     ///
     /// Returns a new `Triangle` instance with the same points, but the points