Add CMYK support to PAM output.
Add CMYK support to PWG output.
Add CMYK and CMYK Alpha colorspaces to mudraw options.
Special case clear_pixmap_with_value for CMYK colorspaces.
Fix bug in gray to cmyk color converters.
Fix segv bug in clear_pixmap_with_value functions.

Image masks don't have a colorspace; check before dereferencing.