We always store CMYK data as YCCK, for two reasons:
1. If we ever want to do subsampling, then doing 2111 or
2112 makes sense with YCCK, while it doesn't make sense
if we store CMYK directly.
2. It forces us to write a color transform header. With a color
transform header, everyone agrees that the CMYK channels should
be stored inverted, while without it behavior between decoders
is inconsistent. (We could write an explicit color transform header
for CMYK too though, but with YCCK it's harder to forget since the
output will look wrong everywhere without it.)
initialize_mcu() grows a full CMYKBitmap override. Some of the
macroblock traversal could probably shared with some kind of
for_all_macroblocks() type function in the future, but the color
conversion math is different enough that this should be a separate
function.
Other than that, we pass around a mode parameter and make a few fuctions
write 4 instead of 3 channels, and that's it.
We use the luminance quantization and huffman tables for the K
channel.
This encoder is very naive as it only output SOF0 images and uses
pre-defined Huffman tables.
There is also a small bug with quantization which make using it
over-degrade the quality.
