Add X, Y, and Z unit vectors

src/basics.cc

@@ -15,6 +15,9 @@
 #pragma mark - Vectors
 
 const Vector3 Vector3::Zero = Vector3();
+const Vector3 Vector3::X = Vector3(1, 0, 0);
+const Vector3 Vector3::Y = Vector3(0, 1, 0);
+const Vector3 Vector3::Z = Vector3(0, 0, 1);
 
 
 /*

src/basics.h

@@ -38,6 +38,9 @@ struct Vector3
     Vector3 &normalize();
 
     static const Vector3 Zero;
+    // Unit vectors in each of the three cartesian directions.
+    static const Vector3 X, Y, Z;
+
     float x, y, z;
 };