Basic rich text

[cmyr]

This is a pair of commits that allow for the creation of a rich text object, that can be displayed in a RawLabel or anything else that uses a TextLayout object.

I don't love this code, but it at least covers the simple case, and I think it is worth checkpointing.

Details: for each annotation type, we maintain a sorted vec of non-overlapping spans. The annotation types in druid are slightly different from what is available in piet; in particular in druid we support FontDescriptor as well as (where possible) KeyOrValue. Druid annotations can be added in any order (not restricted to non-descending start order) and then when we rebuild the layout we resolve them to piet spans and construct the actual layout object.

stuff I don't like:

• layout may not be invalidated correctly if the env changes (this is fixable, just not in here yet)
• no functionality for editing the text; you have to recreate the whole object. I'm still not sure how this should interact with editing.
• default styles are confused; they currently live on the TextLayout, not on the RichText; this should probably change in some way, so that if you're using RichText we ignore anything set on the label itself.

You can play with this at examples/text.rs:

    let text = RichText::new(TEXT.into())
        .with_attribute(0..9, Attribute::text_color(Color::rgb(1.0, 0.2, 0.1)))
        .with_attribute(0..9, Attribute::size(24.0))
        .with_attribute(0..9, Attribute::font_family(FontFamily::SERIF))
        .with_attribute(194..239, Attribute::weight(FontWeight::BOLD))
        .with_attribute(764.., Attribute::size(12.0))
        .with_attribute(764.., Attribute::style(FontStyle::Italic));

[cmyr]

maybe rename this? with_ seems to suggest a builder method.

[Zarenor]

I think from_ would be appropriate here? (It consumes the text, right?)

[raphlinus]

Looks good. I can see a few things that might change long term, but nothing that I would consider blocking now.

[cmyr]

I think there is still some stuff wrong with this PR but I'll open issues for those things, and I think getting it in will unblock some other stuff.