Vector2: add explicit constructor for Vector3

Initialize from the first 2 elements.

matrix/Vector2.hpp

@@ -22,6 +22,7 @@ class Vector2 : public Vector<Type, 2>
 public:
 
     typedef Matrix<Type, 2, 1> Matrix21;
+    typedef Vector<Type, 3> Vector3;
 
     Vector2() = default;
 
@@ -42,6 +43,13 @@ public:
         v(1) = y;
     }
 
+    explicit Vector2(const Vector3 & other)
+    {
+        Vector2 &v(*this);
+        v(0) = other(0);
+        v(1) = other(1);
+    }
+
     Type cross(const Matrix21 & b) const {
         const Vector2 &a(*this);
         return a(0)*b(1, 0) - a(1)*b(0, 0);

test/vector2.cpp

@@ -29,6 +29,11 @@ int main()
     TEST(fabs(f(0) - 4) < 1e-5);
     TEST(fabs(f(1) - 5) < 1e-5);
 
+    Vector3f g(1.23f, 423.4f, 3221.f);
+    Vector2f h(g);
+    TEST(fabs(h(0) - 1.23f) < 1e-5);
+    TEST(fabs(h(1) - 423.4f) < 1e-5);
+
     return 0;
 }