Expose a getter to `ScrollbarPainter._thumbRect`

[angelosilvestre]

Use case

I'd like to write tests that involve dragging the Scrollbar thumb. However, the thumb isn't a dedicated widget, it's just painted inside the ScrollbarPainter. Also, the thumb rect is private. Currently, there is no way to find where the Scrollbar thumb is.

Proposal

Provide a way to allow querying the scrollbar's thumb rectangle.