Unprocessed_raw, masked pixels

Unprocessed_raw is doing its job but when using it for astronomical images the inclusion of masked pixels is a big problem for processing Astronomical images. It makes background detection difficult because it is unknown which pixels have been illuminated and which not. DCRaw doesn't show that behavior.

On my wishlist are the following points:

- Option to exclude the masked pixels. So no dark areas.

- Inclusion of meta-data in the PGM comment section as already proposed at https://github.com/LibRaw/LibRaw/issues/285