Apply preferred font weight when loading variable fonts

[pmnxis]

Previously, when loading a variable font (e.g. via egui::FontData::from_static),
the font was rendered using the default (often the lightest) weight,
ignoring any preferred weight configuration.

This change applies the specified weight to skrifa's Location for the wght axis,
ensuring that variable fonts are rendered with the intended font weight.

Summary

Fixes variable font weight not being applied during rendering. The FontData::weight() method now properly configures the font variation axis.

Changes

• Add location: Location field to FontFace to store variation coordinates
• Pass location parameter through to glyph rendering functions
• Apply weight to skrifa's LocationRef in DrawSettings and HintingInstance

Weight Priority

1. preferred_weight from FontData::weight()
2. OS/2 table's us_weight_class
3. Variable font's fvar default value
4. Location::default()

Related Issue

• add Fontweight to fontid #3218 : Not follow font id, but goal would be same

Todo

• Apply preferred font weight when loading variable fonts
• Add small size variable fonts for docs and egui (need discussion)

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/7790-featurefont-weight-0333
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.

View snapshot changes at kitdiff

[emilk]

Nice - thanks!

[emilk]

Do you want to do this before I hit merge?

[pmnxis]

@emilk
Checkpoint “Add small size variable fonts for docs and egui (need discussion)”,

https://github.com/emilk/egui/pull/7790/files#diff-476cc9a474dfda61ceeb981bd3ea229a1ab4eb65330e74c0128ce257236938c9R177-R187
In this PR , the new docs code uses UbuntuLight instead of some variable font.

Ideally, it would be better to include a variable font directly in the source tree and add
some verification code to ensure that variable fonts actually work as expected.
I have already validated this approach in my own project:
https://github.com/pmnxis/chama-optics

That said, adding variable fonts would significantly increase the source tree size,
so keeping the existing example as-is also seems like a reasonable choice.

[valadaptive]

I'm a bit confused by this PR. When will a variable font's default weight be the "lightest" one? By definition, the default value for any axis should be mapped to 0.0 in variation space, which is what Skrifa uses if you don't specify any coordinates.

This PR changes the font loading process to use the weight value from the OS/2 table if present, but falls back to the default wght axis value from the fvar table if not, which is...exactly what we were doing before.

Can you give examples of variable fonts that default to the lightest weight? Are they fixed if you use the weight from the OS/2 table, or do you still need to manually override the weight when loading them?

[pmnxis]

@valadaptive

Can you give examples of variable fonts that default to the lightest weight?
The Noto CJK variable fonts are an example:
https://github.com/notofonts/noto-cjk/tree/main/google-fonts

After re-examining the OS/2 table, I confirmed that these fonts have
<usWeightClass value="100"/>, which explains why they default to the lightest weight. And I mistaked I didn't checked well it.

That said, there are still cases where manually overriding the font weight is necessary.
In those situations, the function I mentioned in your PR, pub fn weight(self, weight: u16) -> Self is still allowed by your changes, and I’ve confirmed that it correctly covers this use case.

const FONT_NTSANS: BuiltInFonts = BuiltInFonts {
    name: "Noto Sans",
    data: include_bytes!("../../assets/fonts/NotoSansJP[wght].ttf"),
};

// Demonstrates how to replace all fonts.
pub(crate) fn replace_fonts(ctx: &egui::Context) {
    let mut fonts = egui::FontDefinitions::default();

    fonts.font_data.insert(
        FONT_NTSANS.name.to_owned(),
        std::sync::Arc::new(egui::FontData::from_static(FONT_NTSANS.data).weight(400)),
    );
    
    ctx.set_fonts(fonts);
}