Support SVG sources (closes #68)

The fetcher accepts image/svg+xml again and internal/magic detects SVG.
A raster format renders the SVG with librsvg at the density that makes
it cover the output size, so an enlarged SVG stays sharp; the rendered
size is held to MaxInputDimension. orig of an SVG at the orig size is
the SVG unchanged, cached as a variant; at any other size it is
ErrSVGNotOriginalSize, answered 400. The cache reads up to 1024 first
bytes of a file to find its format, as an SVG's root element can come
after a prolog. Tests that pinned SVG as refused now expect it to be
read.

Model: opus-5-5

internal/httpfetcher/httpfetcher.go

@@ -119,6 +119,7 @@ func allowedContentTypes() []string {
 		imagerequest.FormatWebP.MIMEType(),
 		imagerequest.FormatAVIF.MIMEType(),
 		imagerequest.FormatJXL.MIMEType(),
+		imagerequest.FormatSVG.MIMEType(),
 	}
 }
 

internal/httpfetcher/httpfetcher_internal_test.go

@@ -279,7 +279,7 @@ func TestDetectContentTypeFromPath(t *testing.T) {
 		{"foo/bar.gif", imagerequest.FormatGIF.MIMEType()},
 		{"foo/bar.webp", imagerequest.FormatWebP.MIMEType()},
 		{"foo/bar.avif", imagerequest.FormatAVIF.MIMEType()},
-		{"foo/bar.svg", contentTypeSVG},
+		{"foo/bar.svg", imagerequest.FormatSVG.MIMEType()},
 		{"foo/bar.bin", contentTypeOctetStream},
 		{"foo/bar", contentTypeOctetStream},
 	}

internal/httpfetcher/mock.go

@@ -66,11 +66,9 @@ func (m *MockFetcher) Fetch(
 	}, nil
 }
 
-// The content types the mock fetcher gives files in no format pixa reads.
-const (
-	contentTypeSVG         = "image/svg+xml"
-	contentTypeOctetStream = "application/octet-stream"
-)
+// contentTypeOctetStream is the content type the mock fetcher gives files in
+// no format pixa reads.
+const contentTypeOctetStream = "application/octet-stream"
 
 // detectContentTypeFromPath returns the MIME type based on file extension.
 func detectContentTypeFromPath(path string) string {
@@ -90,7 +88,7 @@ func detectContentTypeFromPath(path string) string {
 	case strings.HasSuffix(path, ".jxl"):
 		return imagerequest.FormatJXL.MIMEType()
 	case strings.HasSuffix(path, ".svg"):
-		return contentTypeSVG
+		return imagerequest.FormatSVG.MIMEType()
 	default:
 		return contentTypeOctetStream
 	}