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_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},
 	}