Implement writing of CMYK and fix writing of image alpha channel (bug #32986). * __magick_read__.cc (bitdepth_from_class, init_encode_image): new functions created from pieces of encode_indexed_images () to be used by the other encode image functions. (encode_indexed_images): make use of new bitdepth_from_class(), and init_encode_image() functions. (encode_bool_image): rewritten to match flow of the other encode functions, use fortran_vec for performance, and use only 4th dimension for frames. (encode_uint_image): completely rewritten to identify images of CMYK type and not confuse them with RGB plus alpha channel. Now accepts the alpha channel as separate argument. Image argument must now be of same class as the template. (__magick_write__): changed to match new API for the encode functions. * private/__imwrite__.m: set default and input check for alpha channel option. * imwrite.m: document alpha channel option as separate argument. * NEWS: announce rewrite of the image IO functions and warn about possible backwards incompatibilities.

gnulib-hg = gnulib-hg