Add background styling to span / rich text

[tarkah]

Adds a new text::Background type which can be optionally specified on Span.

rich_text will use this to render background quads for spans where this is set.

A new Paragraph::span_bounds method is added to get the bounds of each span to enabled this.

simplescreenrecorder-2024-07-23_13.37.52.mp4

I'm using this in Halloy:

[derezzedex]

I think we could improve the creation of the text::Background with a few helpers, and also possibly changing it to span::Background.
But looks pretty good overall!

[hecrj]

I simplified some stuff here and there, but looks great! 🥳