Code review comment for lp://staging/~kdub/mir/fix-1670876

> > yeah, good point. Unclear part of API imo, can fix in this MP.
>
> Wrote a test to show how to reset input regions, and its alright without
> further modifications. If the client user calls:
> mir_window_spec_set_input_region(spec, nullptr, 0);
>
> The client code will set the input region field on the modification, and that
> will get passed through to the shell.

Great. Thanks!