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/magic/magic_internal_test.go

@@ -47,9 +47,9 @@ func TestDetectFormat(t *testing.T) {
 		{name: "AVIF", data: avif, wantFormat: imagerequest.FormatAVIF},
 		{name: "AVIF sequence", data: avis, wantFormat: imagerequest.FormatAVIF},
 		{
-			name:    "SVG",
-			data:    []byte(`<svg xmlns="http://www.w3.org/2000/svg"></svg>`),
-			wantErr: ErrUnknownFormat,
+			name:       "SVG",
+			data:       []byte(`<svg xmlns="http://www.w3.org/2000/svg"></svg>`),
+			wantFormat: imagerequest.FormatSVG,
 		},
 		{
 			name:    "unknown format",