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

README.md

@@ -245,7 +245,7 @@ A request whose query string cannot be decoded, or gives any parameter more than
 once, is refused with 400.
 
 - `<format>`: one of `orig` (or `original`), `jpeg` (or `jpg`), `png`, `webp`,
-  `avif`, `gif`, or `auto` (below)
+  `avif`, `jxl` (JPEG XL), `gif`, or `auto` (below)
 - `<size>`: `orig` or `<width>x<height>` (e.g. `800x600`)
 - `sig` and `exp`: the signature and its expiry, needed unless the host is
   allowlisted (see Signature Specification)
@@ -253,23 +253,29 @@ once, is refused with 400.
   both optional (values under Signature Specification). Both are part of what is
   cached, so each value of either is a separate cached image.
 
+The source image may be JPEG, PNG, GIF, WebP, AVIF, JPEG XL or SVG. The
+upstream's `Content-Type` must name its format, and the image's first bytes must
+match it.
+
 With the format `auto`, pixa chooses the format for each request from its
 `Accept` header, in this order:
 
-1. AVIF, when the header names `image/avif`;
-2. WebP, when it names `image/webp`;
-3. JPEG, when the first of `image/jpeg`, `image/*` and `*/*` that it names
+1. JPEG XL, when the header names `image/jxl`;
+2. AVIF, when it names `image/avif`;
+3. WebP, when it names `image/webp`;
+4. JPEG, when the first of `image/jpeg`, `image/*` and `*/*` that it names
    allows it, or when there is no `Accept` header or it is empty.
 
 An entry with `q=0` refuses its format; other `q` values do not change the
-order. AVIF and WebP must be named, as clients that cannot show them also send
-`image/*` and `*/*`. pixa never sends a format the client refused: when the
-header allows none of the three, the answer is 406, and a header that does not
-parse, or has a `q` that is not a number from 0 to 1, is refused with 400. The
-signature, or the token of an encrypted URL, covers `auto` itself, so one URL
-serves every client. Each format chosen is cached as a separate image, and every
-answer that depends on `Accept` (the image, a 304, and the 400 and 406 above)
-carries `Vary: Accept`, so a shared cache keeps the formats apart too.
+order. JPEG XL, AVIF and WebP must be named, as clients that cannot show them
+also send `image/*` and `*/*`. pixa never sends a format the client refused:
+when the header allows none of the four, the answer is 406, and a header that
+does not parse, or has a `q` that is not a number from 0 to 1, is refused
+with 400. The signature, or the token of an encrypted URL, covers `auto` itself,
+so one URL serves every client. Each format chosen is cached as a separate
+image, and every answer that depends on `Accept` (the image, a 304, and the 400
+and 406 above) carries `Vary: Accept`, so a shared cache keeps the formats apart
+too.
 
 An image is served with `Cache-Control: public, max-age=<seconds>, immutable`.
 When the URL has an expiry (an `exp`, or the TTL of an encrypted URL), `max-age`
@@ -350,6 +356,10 @@ turned off.
 - An image with an ICC profile is converted to sRGB first, since clients show an
   image with no profile as sRGB. Colours outside sRGB, such as the most
   saturated ones in a Display P3 photo, are clipped.
+- The one exception is JPEG XL with libvips 8.16 and later: libvips then writes
+  an EXIF block of its own into the image, with its orientation, resolution,
+  size and colour space and fixed defaults, but nothing from the source's EXIF,
+  as govips cannot ask libvips to leave it out.
 
 ### Source Hosts