Gradually make estimated scroll position accurate

[JordanMartinez]

See FXMisc/RichTextFX#98

When laying out cells with varied heights, the scroll bar's value is only an estimate of how far the content has been scrolled, not the actual position. Thus, the goal is to turn this estimated value into the accurate value. Since this is only useful in some use cases, this should be an option, not a requirement.

One solution for achieving this thought of by Tomas himself (from above issue):

One could come up with some sophisticated algorithms, where the scroll position starts with an estimate (as it does now), and then in the background gradually improves the estimate. This would result in some flickering of the scrollbars before reaching the exact value, but that might be acceptable.