[cairo] User font take 2

[Bill Spitzak]

Why not have render_glyph and get_glyph_metrics take a ucs4 value 
instead of an index and get rid of the ucs4_to_index method?

Kristian Høgsberg wrote:

>    unsigned long  (*ucs4_to_index)       (void                 *closure,
>                                           unsigned long         ucs4);
> 
>    cairo_status_t (*get_glyph_metrics)   (void                 *closure,
>                                           unsigned long         index,
>                                           cairo_font_options_t 
> *font_options,
>                                           cairo_text_extents_t *metrics);
> 
>    cairo_status_t (*render_glyph)        (void                 *closure,
>                                           unsigned long         index,
>                                           cairo_font_options_t 
> *font_options,
>                                           cairo_t              *cr);