st: Add support for font-feature-settings

Cantarell now supports tabular figures, which are useful in places
where digits should either align or not use different widths. In
order to allow elements to request the feature, add support for
the corresponding CSS property[0].

[0] https://www.w3.org/TR/css-fonts-3/#font-rend-desc

https://gitlab.gnome.org/GNOME/gnome-shell/issues/34

src/st/st-theme-node.c

@@ -3019,6 +3019,39 @@ st_theme_node_get_font (StThemeNode *node)
   return node->font_desc;
 }
 
+gchar *
+st_theme_node_get_font_features (StThemeNode *node)
+{
+  int i;
+
+  ensure_properties (node);
+
+  for (i = node->n_properties - 1; i >= 0; i--)
+    {
+      CRDeclaration *decl = node->properties[i];
+
+      if (strcmp (decl->property->stryng->str, "font-feature-settings") == 0)
+        {
+          CRTerm *term = decl->value;
+
+          if (!term->next && term->type == TERM_IDENT)
+            {
+              gchar *ident = term->content.str->stryng->str;
+
+              if (strcmp (ident, "inherit") == 0)
+                break;
+
+              if (strcmp (ident, "normal") == 0)
+                return NULL;
+            }
+
+          return (gchar *)cr_term_to_string (term);
+        }
+    }
+
+  return node->parent_node ? st_theme_node_get_font_features (node->parent_node) : NULL;
+}
+
 /**
  * st_theme_node_get_border_image:
  * @node: a #StThemeNode