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/magic/jpegxl_internal_test.go

@@ -0,0 +1,58 @@
+package magic
+
+import "testing"
+
+// testMIMEJXL is the MIME type of JPEG XL.
+const testMIMEJXL = "image/jxl"
+
+// TestDetectFormatJPEGXL verifies that both JPEG XL signatures, that of a
+// bare codestream and that of the container, are detected as image/jxl.
+func TestDetectFormatJPEGXL(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name string
+		data []byte
+	}{
+		{"codestream", pad(0xFF, 0x0A)},
+		{"container", pad(
+			0x00, 0x00, 0x00, 0x0C, 0x4A, 0x58, 0x4C, 0x20, 0x0D, 0x0A, 0x87, 0x0A,
+		)},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			got, err := DetectFormat(tt.data)
+			if err != nil || got != MIMETypeJXL {
+				t.Errorf("DetectFormat() = %q, %v, want %q", got, err, testMIMEJXL)
+			}
+
+			err = ValidateMagicBytes(tt.data, testMIMEJXL)
+			if err != nil {
+				t.Errorf("ValidateMagicBytes(%q) error = %v", testMIMEJXL, err)
+			}
+		})
+	}
+}
+
+// TestJPEGXLMIMEType verifies that image/jxl is a supported input type and
+// maps to and from the format jxl.
+func TestJPEGXLMIMEType(t *testing.T) {
+	t.Parallel()
+
+	if !IsSupportedMIMEType(testMIMEJXL) {
+		t.Errorf("IsSupportedMIMEType(%q) = false", testMIMEJXL)
+	}
+
+	format, ok := MIMEToImageFormat(testMIMEJXL)
+	if format != FormatJXL || !ok {
+		t.Errorf("MIMEToImageFormat(%q) = %q, %v, want %q, true",
+			testMIMEJXL, format, ok, FormatJXL)
+	}
+
+	if got := ImageFormatToMIME(FormatJXL); got != testMIMEJXL {
+		t.Errorf("ImageFormatToMIME(%q) = %q, want %q", FormatJXL, got, testMIMEJXL)
+	}
+}