configure: Use Popen instead of check_output

Fixes configure with Python 2.6

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
filters; Ditherings: Make list of output fmts explicit.

Make the list of output formats explicit so that the code doesn't break
when new pixel that is not RGB nor Gray is added.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
core: Add initial support for CMYK8888.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
loaders: JPG: Add support for loading CMYK images.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>