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

A JPEG XL source, a bare codestream or one in its container, is
fetched, detected and decoded, 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, and auto chooses it first when Accept names
image/jxl.

govips sends libvips a JPEG XL distance with every save, and libvips
then ignores the quality, so q is turned into a distance with libvips'
formula, keeping 100 lossy. govips cannot have libvips strip metadata
from JPEG XL, so it is removed from the image before saving; the
conversion to sRGB moved ahead of the format switch to run first.

Model: opus-5-5

internal/httpfetcher/mock.go

@@ -109,6 +109,8 @@ func detectContentTypeFromPath(path string) string {
 		return contentTypeWebP
 	case strings.HasSuffix(path, ".avif"):
 		return contentTypeAVIF
+	case strings.HasSuffix(path, ".jxl"):
+		return contentTypeJXL
 	case strings.HasSuffix(path, ".svg"):
 		return contentTypeSVG
 	default: