src/svgtiny_css.c: add user handler function

src/svgtiny_css.c: add user handler function

This function needs to be passed to dom_node_set_user_data() in
set_libcss_node_data(), but we couldn't implement it before our
css_select_handler was defined, because it passes the address of
that select handler to css_libcss_node_data_handler().