line2d_vector goes into an infinite loop with negative values.

[nwymyczak]

The following goes into an infinite loop (or seems to be at least) on 0.8.0

let pt = Point { x: 2, y: 2 };
let pt2 = Point { x: 7, y: -4 };
let line = rltk::line2d_vector(pt, pt2);

println!("line: {:?}", line);

[nwymyczak]

I added some print statements to the loop of line2d,

    loop {
        pos += slope;
        let new_point = Point::new(pos.x as i32, pos.y as i32);
        println!("{:?} => {:?}", result[result.len() - 1], new_point);
        let ten_millis = std::time::Duration::from_millis(500);
        std::thread::sleep(ten_millis);
        if result[result.len() - 1] != new_point {
            result.push(new_point);
            if new_point == end {
                // arrived
                break;
            }
        }
    }

looks like the slope causes the loop to overshoot.

(start) Point { x: 2, y: 2 } => (end) Point { x: 7, y: -4 }
steps 7.81025
slope Vec2 { x: 0.6401844, y: -0.76822126 }
Point { x: 2, y: 2 } => Point { x: 3, y: 1 }
Point { x: 3, y: 1 } => Point { x: 3, y: 0 }
Point { x: 3, y: 0 } => Point { x: 4, y: 0 }
Point { x: 4, y: 0 } => Point { x: 5, y: 0 }
Point { x: 5, y: 0 } => Point { x: 5, y: -1 }
Point { x: 5, y: -1 } => Point { x: 6, y: -2 }
Point { x: 6, y: -2 } => Point { x: 6, y: -2 }
Point { x: 6, y: -2 } => Point { x: 7, y: -3 }
Point { x: 7, y: -3 } => Point { x: 8, y: -4 }
Point { x: 8, y: -4 } => Point { x: 8, y: -5 }

[thebracket]

Wow, I'd not considered negative coordinates at all. I'll see if I can get that fixed post 0.8.2.

[thebracket]

This should take care of the problem. I added a unit test for the case, and the other tests still pass. I'll mark it as "awaiting confirmation", feel free to let me know if you have problems with it. Otherwise, I'll close the issue soon.