Implement scroll_by operation for scrollable

`scroll_by` allows scrolling an absolute offset
that is applied to the current scrolling position.

core/src/vector.rs

@@ -20,6 +20,17 @@ impl Vector {
     pub const ZERO: Self = Self::new(0.0, 0.0);
 }
 
+impl<T> std::ops::Neg for Vector<T>
+where
+    T: std::ops::Neg<Output = T>,
+{
+    type Output = Self;
+
+    fn neg(self) -> Self::Output {
+        Self::new(-self.x, -self.y)
+    }
+}
+
 impl<T> std::ops::Add for Vector<T>
 where
     T: std::ops::Add<Output = T>,