Center dock resize handle hitboxes (#9225)

Also, add a `deferred` function which takes an element to paint after
the current element tree.

Release Notes:

- Improved the size and position of the hitbox for resizing left, right,
and bottom panels.
([#8855](https://github.com/zed-industries/zed/issues/8855))

Co-authored-by: Julia <julia@zed.dev>
Co-authored-by: Nathan <nathan@zed.dev>

crates/gpui/src/window/element_cx.rs

@@ -381,8 +381,7 @@ impl<'a> ElementContext<'a> {
 
         let mut sorted_deferred_draws =
             (0..self.window.next_frame.deferred_draws.len()).collect::<SmallVec<[_; 8]>>();
-        sorted_deferred_draws
-            .sort_unstable_by_key(|ix| self.window.next_frame.deferred_draws[*ix].priority);
+        sorted_deferred_draws.sort_by_key(|ix| self.window.next_frame.deferred_draws[*ix].priority);
         self.layout_deferred_draws(&sorted_deferred_draws);
 
         self.window.mouse_hit_test = self.window.next_frame.hit_test(self.window.mouse_position);