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 becomes a distance, keeping 100 lossy. Metadata is removed from
the image before the JPEG XL save, as govips cannot have libvips strip
it, and the image is given 72 dpi so that the EXIF block libvips 8.16
adds holds nothing from the source. The sRGB conversion moved ahead of
the format switch, and a CMYK image with no ICC profile is converted
to sRGB, as libvips cannot save CMYK as JPEG XL.

Model: opus-5-5

internal/imgcache/imgcache.go

@@ -21,11 +21,12 @@ const (
 	FormatPNG      ImageFormat = "png"
 	FormatWebP     ImageFormat = "webp"
 	FormatAVIF     ImageFormat = "avif"
+	FormatJXL      ImageFormat = "jxl"
 	FormatGIF      ImageFormat = "gif"
 
-	// FormatAuto stands for AVIF, WebP or JPEG, chosen for each request
-	// from its Accept header once the URL's signature or token has been
-	// checked; it is never processed or cached as itself.
+	// FormatAuto stands for JPEG XL, AVIF, WebP or JPEG, chosen for each
+	// request from its Accept header once the URL's signature or token has
+	// been checked; it is never processed or cached as itself.
 	FormatAuto ImageFormat = "auto"
 )