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/magic/magic.go

@@ -26,6 +26,7 @@ const (
 	MIMETypeWebP = MIMEType("image/webp")
 	MIMETypeGIF  = MIMEType("image/gif")
 	MIMETypeAVIF = MIMEType("image/avif")
+	MIMETypeJXL  = MIMEType("image/jxl")
 	MIMETypeSVG  = MIMEType("image/svg+xml")
 )
 
@@ -40,6 +41,7 @@ const (
 	FormatPNG      ImageFormat = "png"
 	FormatWebP     ImageFormat = "webp"
 	FormatAVIF     ImageFormat = "avif"
+	FormatJXL      ImageFormat = "jxl"
 	FormatGIF      ImageFormat = "gif"
 )
 
@@ -62,6 +64,11 @@ var (
 	// AVIF uses the ftyp box with brand "avif" or "avis"
 	// Format: size(4 bytes) + "ftyp" + brand(4 bytes)
 	magicFtyp = []byte{0x66, 0x74, 0x79, 0x70} // "ftyp"
+	// JPEG XL is a bare codestream or a codestream in a container
+	magicJXLCodestream = []byte{0xFF, 0x0A}
+	magicJXLContainer  = []byte{
+		0x00, 0x00, 0x00, 0x0C, 0x4A, 0x58, 0x4C, 0x20, 0x0D, 0x0A, 0x87, 0x0A,
+	}
 )
 
 // WebP identifier appears at offset 8 after RIFF header.
@@ -115,6 +122,12 @@ func DetectFormat(data []byte) (MIMEType, error) {
 		}
 	}
 
+	// Check JPEG XL (FF0A, or the 12-byte container signature)
+	if bytes.HasPrefix(data, magicJXLCodestream) ||
+		bytes.HasPrefix(data, magicJXLContainer) {
+		return MIMETypeJXL, nil
+	}
+
 	// Check SVG - look for XML declaration or SVG tag
 	if detectSVG(data) {
 		return MIMETypeSVG, nil
@@ -181,7 +194,8 @@ func normalizeMIMEType(mimeType string) string {
 func IsSupportedMIMEType(mimeType string) bool {
 	normalized := normalizeMIMEType(mimeType)
 	switch MIMEType(normalized) {
-	case MIMETypeJPEG, MIMETypePNG, MIMETypeWebP, MIMETypeGIF, MIMETypeAVIF, MIMETypeSVG:
+	case MIMETypeJPEG, MIMETypePNG, MIMETypeWebP, MIMETypeGIF, MIMETypeAVIF,
+		MIMETypeJXL, MIMETypeSVG:
 		return true
 	default:
 		return false
@@ -226,6 +240,8 @@ func MIMEToImageFormat(mimeType string) (ImageFormat, bool) {
 		return FormatGIF, true
 	case MIMETypeAVIF:
 		return FormatAVIF, true
+	case MIMETypeJXL:
+		return FormatJXL, true
 	case MIMETypeSVG:
 		// SVG has no corresponding output format.
 		return "", false
@@ -247,6 +263,8 @@ func ImageFormatToMIME(format ImageFormat) string {
 		return string(MIMETypeGIF)
 	case FormatAVIF:
 		return string(MIMETypeAVIF)
+	case FormatJXL:
+		return string(MIMETypeJXL)
 	case FormatOriginal:
 		// Original format passes content through unchanged.
 		return mimeOctetStream