Serve and accept JPEG XL as an image format (part of #222)

A JPEG XL source is accepted, and orig of one is JPEG XL. The format
jxl works in plain and encrypted URLs and on the generator page,
served as image/jxl; auto chooses it first when Accept names
image/jxl.

govips sends libvips a JPEG XL distance, which overrides the quality,
so q is turned into a distance, keeping 100 lossy. govips cannot have
libvips strip metadata from JPEG XL, so it is removed from the image
before saving; libvips 8.16 and later still add an EXIF block of their
own, with nothing from the source. The conversion to sRGB moved ahead
of the format switch, and a CMYK image with no ICC profile is
converted to sRGB too, as libvips cannot save CMYK as JPEG XL.

Model: opus-5-5

internal/imgcache/urlparser.go

@@ -275,6 +275,8 @@ func parseFormat(s string) (ImageFormat, error) {
 		return FormatWebP, nil
 	case "avif":
 		return FormatAVIF, nil
+	case "jxl":
+		return FormatJXL, nil
 	case "gif":
 		return FormatGIF, nil
 	case "auto":