Background color is applied in the wrong place when rendered segment type is not a TextExt

[FHannes]

In the RichText demo applying styles works correctly, with the exception of the background color. When the editor just has text, there is no issue. When the text has an image insert before the selection, the application of the background color is negatively offset by the number of images before the selection.

[JordanMartinez]

Meh... I guess my title doesn't quite state the issue correctly either...

Still, the problem occurs in ParagraphText. I think there's a line somewhere in there that only considers TextExt objects when constructing the Path background shape.

[JordanMartinez]

Aye. The issue arises here. We'd need a way to determine whether a given rendered node shares the same background color as another consecutive one while accounting for different subclasses of Node that are used to render the segment on the screen. While this issue only mentions the background color, it could equally apply to the rtfx-specific border and underline styles, too.

Due to accounting for non-text objects, we'd also need to calculate the length of a segment to determine how long the range of that segment's background color or border is. Calling SegmentOps#length on the SEG object would easily calculate its length, but it would mean adding the ops object as one of the constructor parameters of ParagraphBox and ParagraphText.

One way would be to add a lambda (Function) that can be passed an argument and return an Optional<Paint> to determine whether two nodes share the same background color. If the the passed-in object is be a StyledSegment<SEG, S>, this would help us determine the background color for some use cases (e.g. when inline css is used) but not for others (when a style class is used). However, passing in the rendered node with type Node will require instanceOf checks and casting before the background color could be determined. (To get around this, we'd need to use the Visitor pattern and make all rendered nodes implement it. Definitely not something I like).