Exporting CMYK/CMYKA PSD files

Merged Alx Sa requested to merge alxsa-psd-cmyk-export into master

This patch attempts to add the ability to export CMYK/A PSD files. It supports 8 and 16 bit precision (Photoshop itself does not support 32 Bit CMYK images).

The GUI now has an identical "Export as CMYK" option as JPEG and TIFF. It is set to be mutually exclusive to a potential Duotone Export (e.g. you can only select one).

When saving, the header's mode and channel number information are set to CMYK per the Adobe specification. The layer channel information is hardcoded to "4" ("5" if alpha channel exists) since GIMP currently does not provide CMYK channel information. The GeglBuffer data is also converted to CMYK using babl.

Initial test images could be exported and reimported back into GIMP, and also opened properly in Photoshop. ImageMagick also recognized them as CMYK files. Further feedback is appreciated!

Edited by Alx Sa

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