css: Give each style its own variable set

gtk_css_style_print relies on this to limit printing of custom
variables to each styles' own variables. Without this, :root
variables get printed for every single style that doesn't define
its own.

gtk/gtkcssstaticstyle.c

@@ -864,7 +864,10 @@ gtk_css_lookup_resolve (GtkCssLookup      *lookup,
     }
   else if (parent_style && parent_style->variables)
     {
-      style->variables = gtk_css_variable_set_ref (parent_style->variables);
+      g_clear_pointer (&style->variables, gtk_css_variable_set_unref);
+      style->variables = gtk_css_variable_set_new ();
+      gtk_css_variable_set_set_parent (style->variables,
+                                       parent_style->variables);
     }
 
   context.provider = provider;