Layout-dependent methods need to call layout() first.

[JordanMartinez]

A deficiency found in FXMisc/RichTextFX#238.

Basic idea from Tomas:

Methods of VirtualFlow whose result depends on the layout should themselves call layout() as the first thing. [Otherwise, the returned results aren't always valid. (I added this)] Such methods would include getCellIfVisible, visibleCells and hit. The fact that they first call layout() should be documented in their Javadoc.

[TomasMikula]

Thanks for tracking this.

[JordanMartinez]

You're welcome.

[TomasMikula]

Fixed by #20.