fix: svg decoding

There's an occasional bug with the JPEGs within SVGs, in that they don't decode properly. I need to write tests and investigate.

formats/web/postcard.svg.qtpl

···
-{% func SVG(v svgVars) %}
+{%- func SVG(v svgVars) -%}
 <svg
   xmlns="http://www.w3.org/2000/svg"
   {%- code

formats/web/postcard.svg.qtpl.go

···
 //line postcard.svg.qtpl:1
 func StreamSVG(qw422016 *qt422016.Writer, v svgVars) {
 //line postcard.svg.qtpl:1
-	qw422016.N().S(`
-<svg
+	qw422016.N().S(`<svg
   xmlns="http://www.w3.org/2000/svg"
 `)
 //line postcard.svg.qtpl:5

internal/images/decode.go

···
 	{"\xff\xd8", ReadJPEG},
 	{"\x89PNG\r\n\x1a\n", ReadPNG},
 	{"RIFF????WEBPVP8", ReadWebP},
+	{"<svg", ReadSVG},
 }
 
 type readPeeker interface {

internal/images/svg.go

···
+package images
+
+import (
+	"bytes"
+	"encoding/base64"
+	"encoding/xml"
+	"fmt"
+	"image"
+	"io"
+	"strings"
+)
+
+func ReadSVG(r io.Reader) (image.Image, []byte, error) {
+	imgBytes, err := extractImageHref(r)
+	if err != nil {
+		return nil, nil, err
+	}
+	return ReadJPEG(bytes.NewReader(imgBytes))
+}
+
+const base64prefix = "data:image/jpeg;base64,"
+
+func extractImageHref(r io.Reader) ([]byte, error) {
+	decoder := xml.NewDecoder(r)
+
+	for {
+		tok, err := decoder.Token()
+		if err == io.EOF {
+			return nil, fmt.Errorf("no postcard image found in SVG")
+		} else if err != nil {
+			return nil, fmt.Errorf("unable to parse SVG: %w", err)
+		}
+
+		switch se := tok.(type) {
+		case xml.StartElement:
+			if se.Name.Local == "image" {
+				for _, attr := range se.Attr {
+					if attr.Name.Local == "href" {
+						if strings.HasPrefix(attr.Value, base64prefix) {
+							b64 := attr.Value[len(base64prefix):]
+							return base64.StdEncoding.DecodeString(b64)
+						}
+					}
+				}
+			}
+		}
+	}
+}