matrix: fix slice to slice assignment to do deep copy

To fix usage of a.xy() = b.xy() which should copy
the first two elements over into a and not act on a copy of a.

src/lib/matrix/matrix/Slice.hpp

@@ -35,6 +35,8 @@ public:
 		assert(y0 + Q <= N);
 	}
 
+	Slice(const Slice<Type, P, Q, M, N> &other) = default;
+
 	const Type &operator()(size_t i, size_t j) const
 	{
 		assert(i < P);
@@ -52,6 +54,12 @@ public:
 		return (*_data)(_x0 + i, _y0 + j);
 	}
 
+	// Separate function needed otherwise the default copy constructor matches before the deep copy implementation
+	Slice<Type, P, Q, M, N> &operator=(const Slice<Type, P, Q, M, N> &other)
+	{
+		return this->operator=<M, N>(other);
+	}
+
 	template<size_t MM, size_t NN>
 	Slice<Type, P, Q, M, N> &operator=(const Slice<Type, P, Q, MM, NN> &other)
 	{

src/lib/matrix/test/MatrixSliceTest.cpp

@@ -262,3 +262,12 @@ TEST(MatrixSliceTest, Slice)
 	float O_check_data_12 [4] = {2.5, 3, 4, 5};
 	EXPECT_EQ(res_12, (SquareMatrix<float, 2>(O_check_data_12)));
 }
+
+TEST(MatrixSliceTest, XYAssignmentTest)
+{
+	Vector3f a(1, 2, 3);
+	Vector3f b(4, 5, 6);
+	// Assign first two elements from b to first two slot of a
+	a.xy() = b.xy();
+	EXPECT_EQ(a, Vector3f(4, 5, 3));
+}