Support SVG sources (closes #68)

The fetcher accepts image/svg+xml again and internal/magic detects SVG.
A raster format renders the SVG at the least size that covers the
output, held to 8192 on a side, through libvips' thumbnail, which
renders at any scale, so an enlarged SVG stays sharp. An SVG's own size
is not limited; one with a side more than 8192 times the other is
ErrSVGTooThin, answered 415. 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. Render time and memory grow with
what the SVG draws: P1 in TODO.md. Tests that pinned SVG as refused now
expect it to be read.

Model: opus-5-5

internal/imgcache/svg_internal_test.go

@@ -0,0 +1,54 @@
+package imgcache
+
+import (
+	"testing"
+
+	"sneak.berlin/go/pixa/internal/imagerequest"
+)
+
+// testSVG is an SVG with a comment before its root element, so that its
+// first magic.MinMagicBytes bytes do not show its format.
+const testSVG = `<!-- a drawing --><svg xmlns="http://www.w3.org/2000/svg" ` +
+	`width="4" height="4"/>`
+
+// TestSourceAndVariant_ReadSVG checks that a cached SVG source is read as
+// SVG, and a cached SVG variant (orig of an SVG source) as image/svg+xml.
+func TestSourceAndVariant_ReadSVG(t *testing.T) {
+	t.Parallel()
+
+	cache, _ := newEvictionTestCache(t, 1<<30)
+
+	storeEvictionTestSource(t, cache, testHostCDN, "/drawing.svg", []byte(testSVG))
+
+	reader, _, format, err := cache.Source(t.Context(),
+		imagerequest.Source{Host: testHostCDN, Path: "/drawing.svg"})
+	t.Logf("Source() format = %q, error = %v", format, err)
+
+	if err != nil || format != imagerequest.FormatSVG {
+		t.Errorf("Source() = format %q, error %v; want %q", format, err,
+			imagerequest.FormatSVG)
+	}
+
+	if reader != nil {
+		_ = reader.Close()
+	}
+
+	key := imagerequest.VariantKey("aabbccdd3001")
+
+	err = cache.variants.Write(string(key), []byte(testSVG))
+	if err != nil {
+		t.Fatalf("storing the variant file: %v", err)
+	}
+
+	reader, _, contentType, err := cache.Variant(key)
+	t.Logf("Variant() content type = %q, error = %v", contentType, err)
+
+	if err != nil || contentType != "image/svg+xml" {
+		t.Errorf("Variant() = content type %q, error %v; want image/svg+xml",
+			contentType, err)
+	}
+
+	if reader != nil {
+		_ = reader.Close()
+	}
+}