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

internal/imageprocessor/imageprocessor.go

@@ -37,6 +37,24 @@ func initVips() {
 	})
 }
 
+// errNoJPEGXL is returned by CheckJPEGXLSupport.
+var errNoJPEGXL = errors.New("libvips lacks JPEG XL support: install " +
+	"vips-jxl on Alpine, or use a libvips built with libjxl")
+
+// CheckJPEGXLSupport returns an error, naming the fix, when libvips
+// cannot load and save JPEG XL.
+func CheckJPEGXLSupport() error {
+	initVips()
+
+	// govips counts a format as supported when libvips has its loader;
+	// libvips builds the JPEG XL loader and saver together.
+	if !vips.IsTypeSupported(vips.ImageTypeJXL) {
+		return errNoJPEGXL
+	}
+
+	return nil
+}
+
 // Format represents supported output image formats.
 type Format string