Custom default replacement glyphs for font

[Warpten]

Is your feature request related to a problem? Please describe.

There is currently no way to define a custom replacement glyph for user-provided fonts. For example, the font "MaterialSymbolsSharp" can't be used with egui because it does not have the replacement glyphs egui is hardcoded to look for. The font itself seems to load fine; however as soon as I try to use the font the code panics here:

Describe the solution you'd like Add user-specified overrides to FontTweak that replace the glyphs that the library looks for.

Describe alternatives you've considered Besides using another font, there isn't really any option. Even using font fallbacks (for example with RichText::append_to), egui still tries to load the font and panics before falling back.

Additional context Seen on egui 0.27.2 on a custom vulkan backend (although that is unrelated to the issue at hand).

Usage:

// This happens on app start, looks through ./assets/fonts and loads everything:
def.font_data.insert(
    font_name.to_owned(), // MaterialSymbolsSharp
    FontData::from_owned(file_data.unwrap())
);

def.families.entry(value.clone())
    .and_modify(move |value| value.push(font_name.to_owned()))
    .or_insert(vec![font_name.to_owned()]);

def.families.insert(FontFamily::Name(font_name.into()), vec![font_name.to_owned()]);

// Later in the UI
let mut label_job = LayoutJob::default();
RichText::new("HOME")
    .family(FontFamily::Name("MaterialSymbolsSharp".into()))
    .append_to(&mut label_job, &Style::default(), FontSelection::Default, egui::Align::Min);
RichText::new("Home")
    .family(FontFamily::Name("Roboto-Light".into()))
    .append_to(&mut label_job, &Style::default(), FontSelection::Default, egui::Align::Min);

ui.selectable_value(&mut self.active_tab, Tab::Home, label_job);

[Warpten]

Of course I find #3526 just after creating this issue