theme-node: Fix box-shadows for prerendered textures

The material of prerendered backgrounds is now painted in the
rectangle determined by st_theme_node_get_paint_box(). As the
ClutterActorBox returned from that function includes the space
needed to draw the box shadow, the background ends up occluding
the shadow.
As the box shadow is not part of the background, factor out a new
helper function which excludes the box shadow, and use it to
prerender and place the background material.

https://bugzilla.gnome.org/show_bug.cgi?id=641522

src/st/st-theme-node-drawing.c

@@ -748,7 +748,7 @@ st_theme_node_render_background_with_border (StThemeNode *node)
    * may need to create an image bigger than the nodes
    * allocation
    */
-  st_theme_node_get_paint_box (node, &actor_box, &paint_box);
+  st_theme_node_get_background_paint_box (node, &actor_box, &paint_box);
 
   /* translate the boxes so the paint box is at 0,0
   */
@@ -1691,7 +1691,9 @@ st_theme_node_paint (StThemeNode           *node,
         {
           ClutterActorBox paint_box;
 
-          st_theme_node_get_paint_box (node, &allocation, &paint_box);
+          st_theme_node_get_background_paint_box (node,
+                                                  &allocation,
+                                                  &paint_box);
 
           paint_material_with_opacity (node->prerendered_material,
                                        &paint_box,