When loading variable fonts in egui, the first available weight was selected
instead of the font’s default weight, making it impossible to control the
rendered weight.

This change applies the preferred weight (or the font’s default value) to the
`wght` axis, ensuring variable fonts are rendered with the intended weight.

Related PR: emilk/egui#7790

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
- emilk#3218 : Not follow font id, but goal would be same

## Todo
* [x] Apply preferred font weight when loading variable fonts
* [ ] Add small size variable fonts for docs and egui (need discussion)