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

script/bootstrap --cgo installs vips-jxl when the package manager it
uses is apk, as Alpine's vips package lacks JPEG XL support; the nix
and brew libvips include it, as does apt's from Debian 12 and Ubuntu
24.04 on. The runtime stage of the Dockerfile installs vips-jxl too.

imgcache.NewService calls the new imageprocessor.CheckJPEGXLSupport
before it builds the image processor and fails with its error, which
names the fix, so pixad does not start without the support. JPEG XL
becomes the default output later in the issue. New tests check the
support and save an image as JPEG XL with govips and load 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,
@@ -585,8 +588,8 @@ 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")