Install libvips JPEG XL support and require it at startup (part of #222)

script/bootstrap --cgo installs vips-jxl on Alpine, where libvips' JPEG
XL loader and saver are a package of their own; the nix and brew
libvips include them, as does apt's from Debian 12 and Ubuntu 24.04 on.
The runtime stage of the Dockerfile installs vips-jxl too.

imageprocessor.New now returns an error when libvips cannot load and
save JPEG XL, and NewService passes it on, so pixad does not start
without that support. JPEG XL becomes the default output later in the
issue, which a pixad without it could not serve. A new test saves an
image as JPEG XL with govips and loads it back.

Model: opus-5-5

README.md

@@ -89,7 +89,10 @@ another part of pixa failed to stop. A request not finished by then is cut off.
 `docker stop` waits 10 seconds before it kills the container.
 
 Outside Docker, pixa needs libvips (the image has 8.15) and libheif to run, as
-it uses libvips through CGO; building it also needs their development files,
+it uses libvips through CGO. pixad does not start unless libvips has its JPEG XL
+support, which on Alpine is the `vips-jxl` package and which the nix and brew
+packages of libvips include, as do the apt ones from Debian 12 and Ubuntu 24.04
+on. Building pixa also needs the development files of libvips and libheif,
 `pkg-config` and a C compiler. `script/bootstrap --cgo` installs all of these,
 as the `Dockerfile` does where it compiles pixa. Plain `script/bootstrap`, which
 `script/setup` and `script/cibuild` run, installs git, make and Go, and Node,
@@ -582,8 +585,8 @@ provide:
 
 - `script/bootstrap` — install git, make, Go, Node, Yarn and prettier and
   download the Go modules (idempotent); with `--cgo`, the C compiler and the
-  libvips and libheif libraries that compiling pixa needs instead of Node, Yarn
-  and prettier
+  libvips (with its JPEG XL support) and libheif libraries that compiling and
+  testing pixa need instead of Node, Yarn and prettier
 - `script/setup` — make a fresh clone ready for development (bootstrap, then
   install-precommit)
 - `script/projectname` — output the project name ("pixa")