message: matrix.h: fix ambiguous `diag` function (MSVC2017)
  After updating to Visual Studio C++ 15.5.1, calls to `diag` in Mitsuba2's `Transform::perspective` yielded an error (ambiguous call).
diff: @@ -211,7 +211,7 @@ ENOKI_INLINE Matrix diag(const column_t<Matrix> &value) {
  return result;
  }
  -template <typename Matrix>
  +template <typename Matrix, std::enable_if_t<!column_t<Matrix>::IsMatrix, int> = 0>
  ENOKI_INLINE column_t<expr_t<Matrix>> diag(const Matrix &value) {
  column_t<expr_t<Matrix>> result;
  for (size_t i = 0; i < Matrix::Size; ++i)
  