Prepare to use TriMesh in latest experiment

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

@@ -70,6 +70,7 @@ impl Triangulate for FaceApprox {
             let points = triangle.map(|point| point.point_global);
             mesh.triangles.push(MeshTriangle {
                 inner: points.into(),
+                is_internal: false,
                 color,
             });
         }

crates/fj-interop/src/tri_mesh.rs

@@ -50,6 +50,16 @@ pub struct MeshTriangle {
     /// # The triangle
     pub inner: Triangle<3>,
 
+    /// # Track whether this is an internal triangle
+    ///
+    /// Internal triangles are triangles that are inside of the solid body that
+    /// the triangle mesh is a boundary for.
+    ///
+    /// If the triangle mesh is well-formed, then internal triangles are still
+    /// part of a single contiguous outer shell. But this shell might touch
+    /// itself, making the triangles where that is the case internal.
+    pub is_internal: bool,
+
     /// # The color of the triangle
     pub color: Color,
 }