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/handlers/format_auto.go

@@ -14,8 +14,8 @@ import (
 // Errors for an Accept header that an auto URL cannot be served for.
 var (
 	errInvalidAccept = errors.New("invalid Accept header")
-	errNotAcceptable = errors.New(
-		"not acceptable: auto serves image/avif, image/webp or image/jpeg")
+	errNotAcceptable = errors.New("not acceptable: auto serves " +
+		"image/jxl, image/avif, image/webp or image/jpeg")
 )
 
 // chooseAutoFormat replaces the format auto in req with the format
@@ -51,11 +51,11 @@ func (s *Handlers) chooseAutoFormat(
 }
 
 // formatForAccept returns the format an auto URL is served in for the Accept
-// header accept: AVIF when it names image/avif, else WebP when it names
-// image/webp, else JPEG when its most specific entry of image/jpeg, image/*
-// and */* allows it, or when it names nothing. A q of 0 refuses a format.
-// AVIF and WebP must be named, as clients that cannot show them send image/*
-// and */* too.
+// header accept: JPEG XL when it names image/jxl, else AVIF when it names
+// image/avif, else WebP when it names image/webp, else JPEG when its most
+// specific entry of image/jpeg, image/* and */* allows it, or when it names
+// nothing. A q of 0 refuses a format. JPEG XL, AVIF and WebP must be named, as
+// clients that cannot show them send image/* and */* too.
 func formatForAccept(accept string) (imgcache.ImageFormat, error) {
 	qualities, err := parseAccept(accept)
 	if err != nil {
@@ -66,6 +66,10 @@ func formatForAccept(accept string) (imgcache.ImageFormat, error) {
 		return imgcache.FormatJPEG, nil
 	}
 
+	if qualities["image/jxl"] > 0 {
+		return imgcache.FormatJXL, nil
+	}
+
 	if qualities["image/avif"] > 0 {
 		return imgcache.FormatAVIF, nil
 	}