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>