[cairo] user-fonts: unicode_to_glyph

[Jeff Muizelaar]

> So the problem is that the cairo_scaled_font_text_to_glyphs() to the fallback
> font actually positions the glyphs.  So the scaled advance widths are not used.
> 
> There are two ways to fix it:
> 
>  - The easy way is to implement unicode_to_glyph instead of text_to_glyphs.
> This has some negative effects, but definitely not on your use cases.

I'd like to take this approach, however as far as I can tell I need a
cairo_scaled_font_unicode_to_glyph that basically just calls:
(*scaled_font->backend->ucs4_to_index) (scaled_font, unicode);
If so, can we export API for this?

-Jeff