The use of box-shadow on a StWidget that has a background-gradient was
not been rendered correctly, the shadow borders was calculated inside
the st_theme_node_prerender_shadow function and in the case that we've a
prerendered_texture the max_borders was not calculated and are 0.
This patch creates a new static function to compute shadow maximum
borders copying the code from st_theme_node_prerender_shadow, and call
this new method in the case that we've a prerendered_texture.
https://gitlab.gnome.org/GNOME/gnome-shell/issues/1186
cf00231aa8