Update JPEG XL investigation score

[foolip]

From my review in web-platform-tests/interop-jpegxl#1 (comment) I believe an accurate score is 78.26%.

[foolip]

I've filed issues for what it would take to get to 100%:

• ProPhoto RGB is not tested interop-jpegxl#4
• HDR tests are manual interop-jpegxl#5
• Test VarDCT block smaller/larger than allowed by spec interop-jpegxl#6
• Test animated JPEG XL in <img> interop-jpegxl#7
• Test CSS background-image, content, image-set(), list-style-image rendering interop-jpegxl#8