Math: overload RectangularMatrix::diagonal() in subclasses.

...to return proper type.

src/Math/RectangularMatrix.h

@@ -542,6 +542,9 @@ extern template Corrade::Utility::Debug MAGNUM_EXPORT operator<<(Corrade::Utilit
     }                                                                       \
     static const __VA_ARGS__& from(const T* data) {                         \
         return *reinterpret_cast<const __VA_ARGS__*>(data);                 \
+    }                                                                       \
+    constexpr static __VA_ARGS__ fromDiagonal(const Vector<Math::RectangularMatrix<cols, rows, T>::DiagonalSize, T>& diagonal) { \
+        return Math::RectangularMatrix<cols, rows, T>::fromDiagonal(diagonal); \
     }                                                                       \
                                                                             \
     __VA_ARGS__ operator-() const {                                         \

src/Math/Test/RectangularMatrixTest.cpp

@@ -444,6 +444,7 @@ void RectangularMatrixTest::subclassTypes() {
     const Float* const cdata = nullptr;
     CORRADE_VERIFY((std::is_same<decltype(Mat2x2::from(data)), Mat2x2&>::value));
     CORRADE_VERIFY((std::is_same<decltype(Mat2x2::from(cdata)), const Mat2x2&>::value));
+    CORRADE_VERIFY((std::is_same<decltype(Mat2x2::fromDiagonal({})), Mat2x2>::value));
 
     /* Const operators */
     const Mat2x2 c;
@@ -479,6 +480,9 @@ void RectangularMatrixTest::subclass() {
     CORRADE_COMPARE(Mat2x2::from(cdata), Mat2x2(Vector2(1.0f, -2.0f),
                                                 Vector2(3.0f, -4.5f)));
 
+    CORRADE_COMPARE(Mat2x2::fromDiagonal({1.0f, -2.0f}), Mat2x2(Vector2(1.0f,  0.0f),
+                                                                Vector2(0.0f, -2.0f)));
+
     const Mat2x2 a(Vector2(1.0f, -3.0f),
                    Vector2(-3.0f, 1.0f));
     CORRADE_COMPARE(-a, Mat2x2(Vector2(-1.0f, 3.0f),