cssbgsizevalue: Avoid allocating new value if not needed

gtk/gtkcssbgsizevalue.c

@@ -60,6 +60,16 @@ gtk_css_value_bg_size_compute (GtkCssValue             *value,
   if (value->y)
     y = _gtk_css_value_compute (value->y, property_id, provider, style, parent_style);
 
+  if (x == value->x && y == value->y)
+    {
+      if (x)
+        _gtk_css_value_unref (x);
+      if (y)
+        _gtk_css_value_unref (y);
+
+      return _gtk_css_value_ref (value);
+    }
+
   return _gtk_css_bg_size_value_new (value->x ? x : NULL,
                                      value->y ? y : NULL);
 }