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/httpfetcher/jpegxl_internal_test.go

@@ -0,0 +1,20 @@
+package httpfetcher
+
+import "testing"
+
+// TestJPEGXLContentType verifies that the fetcher accepts an upstream answer
+// of type image/jxl by default, and that the mock fetcher serves a .jxl file
+// as image/jxl.
+func TestJPEGXLContentType(t *testing.T) {
+	t.Parallel()
+
+	const jxlType = "image/jxl"
+
+	if !New(DefaultConfig()).isAllowedContentType(jxlType) {
+		t.Errorf("isAllowedContentType(%q) = false, want true", jxlType)
+	}
+
+	if got := detectContentTypeFromPath("images/photo.jxl"); got != jxlType {
+		t.Errorf("detectContentTypeFromPath(photo.jxl) = %q, want %q", got, jxlType)
+	}
+}