Text background colors

[ickshonpe]

Objective

Add background colors for text.

Fixes #18889

Solution

New component TextBackgroundColor, add it to any UI Text or TextSpan entity to add a background color to its text.
New field on TextLayoutInfo section_rects holds the list of bounding rects for each text section.

The bounding rects are generated in TextPipeline::queue_text during text layout, extract_text_background_colors extracts the colored background rects for rendering.

Didn't include Text2d support because of z-order issues.

The section rects can also be used to implement interactions targeting individual text sections.

Testing

Includes a basic example that can be used for testing:

cargo run --example text_background_colors

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Showcase

Using a proportional font with kerning the results aren't so tidy (since the bounds of adjacent glyphs can overlap) but it still works fine:

[Olle-Lukowski]

Other than my one comment this looks great!

[Henauxg]

I'm not very familiar with render entity extraction so my review of extract_text_background_colors was pretty basic.

In queue_text, we could optionally move the computation of section_rects in the already existing main glyphs loop to avoid having a second linear scan of all the glyphs.