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; 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.

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"
 )