mixed backend rely on __getattr__ method

In the current implementation, when `MixedRenerer._set_current_renderer` is called, it replaces self's methods (e.g., draw_path, etc.) with the methods of the given render where the list of methods to be replaced is hard-coded. Unless there are severe disadvantages, I propose an alternative of using `__getattr__` method, which is much simpler (at least to me).