Reworked Scrollable to account for lack of widget order guarantees.

Fixed thumb "snapping" bug on scrollable when cursor is out of bounds.

native/src/widget/operation/scrollable.rs

@@ -1,19 +1,19 @@
 //! Operate on widgets that can be scrolled.
 use crate::widget::{Id, Operation};
-use iced_core::Vector;
+use iced_core::Point;
 
 /// The internal state of a widget that can be scrolled.
 pub trait Scrollable {
-    /// Snaps the scroll of the widget to the given `percentage`.
-    fn snap_to(&mut self, percentage: Vector<f32>);
+    /// Snaps the scroll of the widget to the given `percentage` along the horizontal & vertical axis.
+    fn snap_to(&mut self, percentage: Point);
 }
 
 /// Produces an [`Operation`] that snaps the widget with the given [`Id`] to
 /// the provided `percentage`.
-pub fn snap_to<T>(target: Id, percentage: Vector<f32>) -> impl Operation<T> {
+pub fn snap_to<T>(target: Id, percentage: Point) -> impl Operation<T> {
     struct SnapTo {
         target: Id,
-        percentage: Vector<f32>,
+        percentage: Point,
     }
 
     impl<T> Operation<T> for SnapTo {