djpegli should decode xyb-jpeg to rgb-png

[sommio]

Describe the bug
djpegli should decode xyb-jpeg to rgb-png to ensure that most programs can read it.

To Reproduce

❯ cjpegli --xyb in.png out.jpeg
Read 1920x1080 image, 2048379 bytes.
Encoding [XYB d1.000 AQ p2 OPT]
Compressed to 187502 bytes (0.723 bpp).
1920 x 1080,  52.273 MP/s [52.27, 52.27], , 1 reps, 1 threads.

❯ djpegli out.jpeg out.png
Read 187502 compressed bytes.
1920 x 1080,  56.085 MP/s [56.09, 56.09], , 1 reps, 1 threads.

❯ ffmpeg -i out.png -i in.png -filter_complex libvmaf -f null -
[Parsed_libvmaf_0 @ 0x5d4fcc17d600] VMAF score: 8.031800

Screenshots

Environment

• OS: Arch Linux + CachyOS&ALHP Repository
• Compiler version: GCC 13.2.1
• CPU type: x86_64-v3
• cjxl/djxl version string: v0.10.2 e148959 [AVX2,SSE4,SSE2]

[RieGo]

I've also run into this problem.
I assume it wouldn't be a lossless decode, if you convert the colorspace.
Maybe an additional option for this would be a good way to deal with this?

[sommio]

It is lossy encoding, using xyb is just for encoding efficiency. So decoding to rgb should be the default.