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

A JPEG XL source is accepted, 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; auto chooses it first when Accept names
image/jxl.

govips sends libvips a JPEG XL distance, which overrides the quality,
so q is turned into a distance, 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, and a CMYK image with no ICC profile is
converted to sRGB too, as libvips cannot save CMYK as JPEG XL.

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()
+		}
+	}
+}