No need to specify renderer for get_tightbbox and get_window_extent#

The get_tightbbox and get_window_extent methods no longer require the renderer kwarg, saving users from having to querry it from fig.canvas.get_renderer. If the renderer kwarg is not supplied these methods first check if there is a cached renderer from a previous draw and use that. If there is no cahched renderer, then the methods will use fig.canvas.get_renderer() as a fallback.