st-theme-node: Add support for inset box-shadows

Implement inset box-shadows as in the CSS3 draft[0]. As the shadow
should appear beneath the node's border, we pick the slow cairo based
rendering path (though a cogl based path could be added in case the
node has a solid background with no borders).

[0] http://www.w3.org/TR/css3-background/#box-shadow

https://bugzilla.gnome.org/show_bug.cgi?id=642334

src/st/st-theme-node.c

@@ -2886,14 +2886,6 @@ st_theme_node_get_box_shadow (StThemeNode *node)
                                    FALSE,
                                    &shadow))
     {
-      if (shadow->inset)
-        {
-          g_warning ("Inset shadows are not implemented for the box-shadow "
-                     "property");
-          st_shadow_unref (shadow);
-          shadow = NULL;
-        }
-
       node->box_shadow = shadow;
 
       return node->box_shadow;