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/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":