slider: Notify on bound update

[CherryWorm]

The slider has the same issue that was fixed in #1588. When resizing, the state isn't notified when the bounds change, leading to the slider not updating until it is rendered again:

Recording.2025-11-15.105802.mp4

Again, I believe the defer is necessary here. Only notifying without deferring does not change this behavior. With the deferred notify, this behavior is fixed:

Recording.2025-11-15.110022.mp4

[huacnlee]

I understand that this is indeed a problem, but using defer to fix it is not the right approach.

Have you tried cargo run -- slider in GPUI Component story, I'm seeing is that there is a slight delay after resizing, but not as slow as when you resize the video.