Add support for defining XYB JPEGXL colorspace on decoding.

[Philbrick]

Fantastic library. JPEGXL decoder appear to be missing methods to define target colorspace when decoding XYB encoded JPEGXL.

[cgohlke]

Do you mean setting the target color profile, which includes the color space, via JxlDecoderSetOutputColorProfile and JxlDecoderSetCms? It doesn't look that straightforward and libjxl only supports RGB and Gray color spaces. The encoder might need support for color profiles too. Anyway, I'll wait for the next release, libjxl 1.0, before considering this.