Broken but funny bouncing.

src/direction.rs

@@ -8,4 +8,9 @@ impl Direction {
     pub fn new(x: isize, y: isize) -> Self {
         Self { x, y }
     }
+
+    pub fn invert(&mut self) {
+        self.x *= -1;
+        self.y *= -1;
+    }
 }