Stub get_fallbacks

I'm looking for the fallback implementation, but I seen this:

/// Get font fallback results for the given text and locale.
///
/// Note: this is currently just a stub implementation, a proper implementation
/// would likely use FontConfig, at least on Linux. It's not clear what a
/// FreeType loader with a non-FreeType source should do.
fn get_fallbacks(&self, text: &str, _locale: &str) -> FallbackResult<Font> {
    warn!("unsupported");
    FallbackResult {
        fonts: Vec::new(),
        valid_len: text.len(),
    }
}

And blame tells me that It's been added 5 years ago by comments, but It seems
like no body is working on this. I hope it can be implemented.

Start with the get_fallbacks entry point and trace how fallback results are expected to be used. Investigate the noted FontConfig approach on Linux and decide how a FreeType loader with a non-FreeType source should behave. Done means the stub is replaced with a proper fallback implementation and the unresolved source behavior has been settled.