Function fz_append_printf 

pub unsafe extern "C" fn fz_append_printf(
    ctx: *mut fz_context,
    buffer: *mut fz_buffer,
    fmt: *const c_char,
    ...
)

fz_append_printf: Format and append data to buffer using
printf-like formatting (see [fz_vsnprintf]).

The buffer will automatically grow as required.