Cleaned up namespaces re: PR comments.

graphics/src/triangle.rs

@@ -7,6 +7,8 @@ pub struct Mesh2D {
     /// The vertices of the mesh
     pub vertices: Vec<Vertex2D>,
     /// The list of vertex indices that defines the triangles of the mesh.
+    ///
+    /// Therefore, this list should always have a length that is a multiple of 3.
     pub indices: Vec<u32>,
 }