Serve and accept JPEG XL as an image format (part of #222)

A JPEG XL source, a bare codestream or one in its container, is
fetched, detected and decoded, 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, and auto chooses it first when Accept names
image/jxl.

govips sends libvips a JPEG XL distance with every save, and libvips
then ignores the quality, so q is turned into a distance with libvips'
formula, 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.

Model: opus-5-5

internal/server/jpegxl_internal_test.go

@@ -0,0 +1,72 @@
+package server
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"testing"
+
+	"github.com/davidbyttow/govips/v2/vips"
+)
+
+// TestImageProxyFlowJPEGXL requests the JPEG XL testdata/red.jxl, 64x48 and
+// made with libvips, through pixa's router, upstream fetcher, image processor
+// and disk cache: resized as PNG, resized as JPEG XL, and at its own size and
+// format, which is JPEG XL. The second request for each URL is a cache hit.
+func TestImageProxyFlowJPEGXL(t *testing.T) {
+	t.Parallel()
+
+	source, err := os.ReadFile("testdata/red.jxl")
+	if err != nil {
+		t.Fatalf("reading the test image: %v", err)
+	}
+
+	upstream := httptest.NewServer(http.HandlerFunc(
+		func(w http.ResponseWriter, _ *http.Request) {
+			w.Header().Set("Content-Type", "image/jxl")
+			_, _ = w.Write(source)
+		}))
+	t.Cleanup(upstream.Close)
+
+	s, _, _ := startImageProxy(t, upstream)
+
+	tests := []struct {
+		sizeFormat    string // the <size>.<format> part of the image URL
+		contentType   string
+		imageType     vips.ImageType
+		width, height int
+	}{
+		{"32x24.png", "image/png", vips.ImageTypePNG, 32, 24},
+		{"32x24.jxl", "image/jxl", vips.ImageTypeJXL, 32, 24},
+		{"orig.orig", "image/jxl", vips.ImageTypeJXL, 64, 48},
+	}
+
+	for _, tt := range tests {
+		target := "/v1/image/" + upstreamHost + "/red.jxl/" + tt.sizeFormat
+
+		for _, wantCache := range []string{"MISS", "HIT"} {
+			rec := getImage(t, s, target)
+			gotType := rec.Header().Get("Content-Type")
+			gotCache := rec.Header().Get("X-Pixa-Cache")
+
+			if gotType != tt.contentType || gotCache != wantCache {
+				t.Errorf("%s: %s, X-Pixa-Cache %s, want %s, %s",
+					target, gotType, gotCache, tt.contentType, wantCache)
+			}
+
+			img, err := vips.NewImageFromBuffer(rec.Body.Bytes())
+			if err != nil {
+				t.Fatalf("%s: loading the image: %v", target, err)
+			}
+
+			if img.Format() != tt.imageType ||
+				img.Width() != tt.width || img.Height() != tt.height {
+				t.Errorf("%s: %s %dx%d, want %s %dx%d", target,
+					vips.ImageTypes[img.Format()], img.Width(), img.Height(),
+					vips.ImageTypes[tt.imageType], tt.width, tt.height)
+			}
+
+			img.Close()
+		}
+	}
+}