Make MonitorId::get_position() return (i32, i32) instead of (u32, u32) because it can be negative on Windows (#324)

src/platform/linux/x11/window.rs

@@ -332,6 +332,8 @@ impl Window2 {
         let mut find = default;
         for monitor in monitors {
             let (mx, my) = monitor.get_position();
+            let mx = mx as u32;
+            let my = my as u32;
             let (mw, mh) = monitor.get_dimensions();
             let (mxo, myo) = (mx+mw-1, my+mh-1);
             let (ox, oy) = (cmp::max(wx, mx), cmp::max(wy, my));