st: Fix box-shadow drawing with prerendered_texture

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.

See #1186 (closed)