Display P3 and Rec. 2020 RGB are not properly tested

[foolip]

Per @foolbar in #4 (comment):

Note wide-gamut-display-p3.html and wide-gamut-rec2020.html are just placeholders, and sunset_logo.jxl in wide-gamut-adobe-rgb.html is a sRGB (instead of Adobe RGB) image, so this isn't the only one left.

@hjanuschka can you take a look at this?

[hjanuschka]

I sent Chromium CL 8308232 to replace the misleading coverage. Display P3 now uses the existing CICP fixture, while Adobe RGB and Rec. 2020 use genuine ICC-tagged JPEG XL fixtures.

The references use independent CSS Color 4 colors (color(display-p3 ...), color(a98-rgb ...), and color(rec2020 ...)) instead of preconverted sRGB PNGs. Each fixture contains four color bands, including values outside sRGB, so an implementation that ignores the image color space will fail.

All three reftests pass in Chromium.

RFC 242 is still needed to preserve and compare wide-gamut samples in screenshot artifacts. This CL does not replace that work; it adds decoder/rendering equivalence coverage using the current reftest infrastructure.

[hjanuschka]

CL landed