skip windows that have no nodata on edges

[perrygeo]

Where does this check happen? in the window function itself or in the NodataPoolMan?

[perrygeo]

Looks like this fits best in compute_window_rgba

Also, the all_valid_edge function will need to be aware of padding.

[perrygeo]

Fixed in 682de4e (close when PR #35 is merged)