We want to render a background *and* the current color (if there is one). This also adds a custom function gtk_render_add_content_path() which adds the path of the current content area to a cairo_t.
No need to call the private versions anymore now that the real ones don't call into the theming engine.
GtkThemingEngine just always calls gtk_do_render_foo(engine->priv->context, ...) now. Other than that, the code is unchanged.
