LibGfx/JPEGWriter: Make it possible to write CMYKBitmaps

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.

Userland/Libraries/LibGfx/ImageFormats/JPEGWriter.h

@@ -21,6 +21,7 @@ public:
     using Options = JPEGEncoderOptions;
 
     static ErrorOr<void> encode(Stream&, Bitmap const&, Options const& = {});
+    static ErrorOr<void> encode(Stream&, CMYKBitmap const&, Options const& = {});
 
 private:
     JPEGWriter() = delete;