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

script/bootstrap

@@ -14,11 +14,12 @@
 #                            script/fmt-check: all the host needs, as
 #                            the checks compile pixa in Docker
 #   script/bootstrap --cgo   git, make, Go, and a C compiler and the
-#                            CGO image libraries (pkg-config, vips,
-#                            libheif) for the govips bindings instead
-#                            of Node: to compile pixa, in the
-#                            Dockerfile's test phase and build stage,
-#                            which format nothing
+#                            CGO image libraries (pkg-config, vips
+#                            with its JPEG XL support, libheif) for
+#                            the govips bindings instead of Node: to
+#                            compile pixa, in the Dockerfile's test
+#                            phase and build stage, which format
+#                            nothing
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
@@ -150,6 +151,12 @@ ensure_cgo_deps() {
     if ! pkg-config --exists vips; then
         pkg_install vips libvips-dev vips vips-dev
     fi
+    # libvips' JPEG XL loader and saver are in the nix and brew vips
+    # packages, and in apt's from Debian 12 and Ubuntu 24.04 on, but in a
+    # package of their own on Alpine.
+    if command -v apk >/dev/null 2>&1 && ! apk info -e vips-jxl >/dev/null; then
+        apk add --no-cache vips-jxl
+    fi
     if ! pkg-config --exists libheif; then
         pkg_install libheif libheif-dev libheif libheif-dev
     fi