message: add more Mat tests
diff: @@ -456,3 +456,15 @@ fn mat_iterator() -> Result<()> {
  assert!(!iter.has_elements());
  Ok(())
  }
  +
  +#[test]
  +fn mat_locate_roi() -> Result<()> {
  + let mat = Mat::from_slice(&[1, 2, 3, 4])?;
  + let roi = Mat::roi(&mat, Rect::new(1, 0, 2, 1))?;
  + let mut sz = Size::default();
  + let mut ofs = Point::default();
  + roi.locate_roi(&mut sz, &mut ofs)?;
  + assert_eq!(sz, Size::new(4, 1));
  + assert_eq!(ofs, Point::new(1, 0));
  + Ok(())
  +}
  