Function rail_caption_lines 

pub fn rail_caption_lines(
    theme: &Theme,
    text_renderer: &mut TextRenderer,
    label: &str,
    max_w: f64,
) -> Vec<String>

A rail caption broken into at most two lines of max_w: at a space when there is one, inside the word (with a hyphen) when one word is too long for a line; the second line elides.