add coloring per text section in gizmos

[nuts-rice]

Objective

• Resolves bevy_gizmos: Multicolored text support. #23002

Solution

• Add StrokeTextLayout::render_with_color_fn() to return polyline and color pairing
• Add text_sections() to build concat string consisting of sections of character-coloring pair
• Associated helper fn for yielding tuple of (char, Color)
• replace text field in StrokeTextLayout with sections that is an iterable &'a [(&'a str, Color)]

Testing

• Ran cargo test for gizmos
• Prolly could use an example

---

Showcase

Example Usage:

gizmos.text_sections(iso, [
      ("Hello ", Color::WHITE),
      ("World!", Color::srgb(1., 0., 0.)),
  ], 25., Vec2::ZERO);

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[ickshonpe]

Yeah this is pretty good, the only problem is the extra allocations, which should be avoidable if StrokeTextLayout's text: &str field is replaced with a slice or iterator. Then make a function, something like:

fn colored_chars(s: &[(&str, Color)]) -> impl Iterator<Item = (char, Color)> {
    s.into_iter()
        .map(|(t, color)| t.chars().map(|c| (c, *color)))
        .flatten()
}

that maps the sections to an iterator that yields the individual chars and their colors for rendering.

[ickshonpe]

Sorry was really tired last night and my review comments were kind of rambling, hopefully you get the idea though.

[nuts-rice]

you're alright, gonna resolve them soon

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[ickshonpe]

Looks good now, everything seems to work:

Just needs the text_gizmos.md release note to be updated to include these changes, and I think the uncolored render function should be removed as not useful.

[kfc35]

I didn’t spot any problems

You can update your PR description if you’d like to remove the mention of the specialized render_with_color_fn function which has since been removed