Add jxl cmyk support

[jcupitt]

I had a quick go, but it fails with:

$ vips copy cmyk.jpg x.jxl
attaching 961644 bytes of ICC
attaching exif-data ..
JxlBasicInfo:
    have_container = 0
    xsize = 1450
    ysize = 2048
    bits_per_sample = 8
    exponent_bits_per_sample = 0
    intensity_target = 0
    min_nits = 0
    relative_to_max_display = 0
    linear_below = 0
    uses_original_profile = 0
    have_preview = 0
    have_animation = 0
    orientation = 1
    num_color_channels = 3
    num_extra_channels = 1
    alpha_bits = 8
    alpha_exponent_bits = 0
    alpha_premultiplied = 0
    preview.xsize = 0
    preview.ysize = 0
    animation.tps_numerator = 10
    animation.tps_denominator = 1
    animation.num_loops = 0
    animation.have_timecodes = 0
JxlPixelFormat:
    num_channels = 4
    data_type = JXL_TYPE_UINT8
    endianness = 0
    align = 0
JxlEncoderFrameSettings:
    tier = 0
    distance = 2.35
    effort = 7
    lossless = 0
./lib/jxl/encode.cc:820: Extra channel 0 is not initialized
JXL_ENC_ERROR

ie. we are passing a 3 channel (CMY) buffer with 1 extra channel (K).

We use JxlEncoderSetExtraChannelInfo() to tag the extra channel as black. I'd think this ought to work, how do we encode an interleaved CMYK image?

[jcupitt]

This pr is for #3880

[jcupitt]

I asked on the libjxl discord and they say CMYK needs to be encoded as CMY plus a separate K channel, so we'll need quite a bit of code to break off the K and compress the extra channel. Let's park this PR for now.

[jcupitt]

Thinking a little more, we'll need to:

• render the whole CMYK (and possibly alpha?) image to a huge memory array
• break it into separate CMY and KA arrays
• encode the CMY, then encode the KA

So that's at least 2x the memory use. We should probably do progressive write before returning to this PR, we might be able to avoid the extra huge memory allocations.

[kleisauke]

I remembered a similar issue when loading a CMYK + Alpha image, see: #3623 (comment).

Furthermore, it looks like cjxl also fails to encode CMYK images: libjxl/libjxl#2126.

[jcupitt]

I've tagged this as blocked upstream since chunked jxl write needs doing first.