Sets the scaled-font initialization function of a user-font.
See cairo_user_scaled_font_init_func_t for details of how the callback
works.
The font-face should not be immutable or a CAIRO_STATUS_USER_FONT_IMMUTABLE
error will occur. A user font-face is immutable as soon as a scaled-font
is created from it.
Since 1.8
Sets the scaled-font initialization function of a user-font. See cairo_user_scaled_font_init_func_t for details of how the callback works. The font-face should not be immutable or a CAIRO_STATUS_USER_FONT_IMMUTABLE error will occur. A user font-face is immutable as soon as a scaled-font is created from it. Since 1.8