resources: Cache Exif data to disk

```bash
name                        old time/op    new time/op    delta
ImageExif/Cold_cache-4         312µs ±28%     355µs ± 7%     ~     (p=0.343 n=4+4)
ImageExif/Cold_cache,_10-4     479µs ± 6%     546µs ± 0%  +13.91%  (p=0.029 n=4+4)
ImageExif/Warm_cache-4         272µs ± 1%      81µs ± 5%  -70.30%  (p=0.029 n=4+4)

name                        old alloc/op   new alloc/op   delta
ImageExif/Cold_cache-4         151kB ± 0%     161kB ± 0%   +6.46%  (p=0.029 n=4+4)
ImageExif/Cold_cache,_10-4     179kB ± 0%     189kB ± 0%   +5.49%  (p=0.029 n=4+4)
ImageExif/Warm_cache-4         151kB ± 0%      13kB ± 0%  -91.52%  (p=0.029 n=4+4)

name                        old allocs/op  new allocs/op  delta
ImageExif/Cold_cache-4         1.03k ± 0%     1.21k ± 0%  +17.78%  (p=0.029 n=4+4)
ImageExif/Cold_cache,_10-4     1.65k ± 0%     1.83k ± 0%  +11.09%  (p=0.029 n=4+4)
ImageExif/Warm_cache-4         1.03k ± 0%     0.28k ± 0%  -72.40%  (p=0.029 n=4+4)
```

Fixes #6291

2 files changed, 58 insertions, 7 deletions

diff --git a/resources/images/exif/exif.go b/resources/images/exif/exif.go
index 7a3c982c1..b5161f770 100644
--- a/resources/images/exif/exif.go
+++ b/resources/images/exif/exif.go
@@ -24,6 +24,8 @@ import (
 	"unicode"
 	"unicode/utf8"
 
+	"github.com/bep/tmc"
+
 	_exif "github.com/rwcarlsen/goexif/exif"
 	"github.com/rwcarlsen/goexif/tiff"
 )
@@ -31,10 +33,10 @@ import (
 const exifTimeLayout = "2006:01:02 15:04:05"
 
 type Exif struct {
-	Lat    float64
-	Long   float64
-	Date   time.Time
-	Values map[string]interface{}
+	Lat  float64
+	Long float64
+	Date time.Time
+	Tags Tags
 }
 
 type Decoder struct {
@@ -139,7 +141,7 @@ func (d *Decoder) Decode(r io.Reader) (ex *Exif, err error) {
 		return
 	}
 
-	ex = &Exif{Lat: lat, Long: long, Date: tm, Values: walker.vals}
+	ex = &Exif{Lat: lat, Long: long, Date: tm, Tags: walker.vals}
 
 	return
 }
@@ -240,3 +242,30 @@ func nullString(in []byte) string {
 
 	return ""
 }
+
+var tcodec *tmc.Codec
+
+func init() {
+	var err error
+	tcodec, err = tmc.New()
+	if err != nil {
+		panic(err)
+	}
+}
+
+type Tags map[string]interface{}
+
+func (v *Tags) UnmarshalJSON(b []byte) error {
+	vv := make(map[string]interface{})
+	if err := tcodec.Unmarshal(b, &vv); err != nil {
+		return err
+	}
+
+	*v = vv
+
+	return nil
+}
+
+func (v Tags) MarshalJSON() ([]byte, error) {
+	return tcodec.Marshal(v)
+}
diff --git a/resources/images/exif/exif_test.go b/resources/images/exif/exif_test.go
index eee60c089..c3cfad1cc 100644
--- a/resources/images/exif/exif_test.go
+++ b/resources/images/exif/exif_test.go
@@ -14,12 +14,15 @@
 package exif
 
 import (
+	"encoding/json"
+	"math/big"
 	"os"
 	"path/filepath"
 	"testing"
 	"time"
 
 	"github.com/gohugoio/hugo/htesting/hqt"
+	"github.com/google/go-cmp/cmp"
 
 	qt "github.com/frankban/quicktest"
 )
@@ -40,16 +43,24 @@ func TestExif(t *testing.T) {
 	c.Assert(x.Lat, qt.Equals, float64(36.59744166666667))
 	c.Assert(x.Long, qt.Equals, float64(-4.50846))
 
-	v, found := x.Values["LensModel"]
+	v, found := x.Tags["LensModel"]
 	c.Assert(found, qt.Equals, true)
 	lensModel, ok := v.(string)
 	c.Assert(ok, qt.Equals, true)
 	c.Assert(lensModel, qt.Equals, "smc PENTAX-DA* 16-50mm F2.8 ED AL [IF] SDM")
 
-	v, found = x.Values["DateTime"]
+	v, found = x.Tags["DateTime"]
 	c.Assert(found, qt.Equals, true)
 	c.Assert(v, hqt.IsSameType, time.Time{})
 
+	// Verify that it survives a round-trip to JSON and back.
+	data, err := json.Marshal(x)
+	c.Assert(err, qt.IsNil)
+	x2 := &Exif{}
+	err = json.Unmarshal(data, x2)
+
+	c.Assert(x2, eq, x)
+
 }
 
 func TestExifPNG(t *testing.T) {
@@ -81,3 +92,14 @@ func BenchmarkDecodeExif(b *testing.B) {
 		f.Seek(0, 0)
 	}
 }
+
+var eq = qt.CmpEquals(
+	cmp.Comparer(
+		func(v1, v2 *big.Rat) bool {
+			return v1.RatString() == v2.RatString()
+		},
+	),
+	cmp.Comparer(func(v1, v2 time.Time) bool {
+		return v1.Unix() == v2.Unix()
+	}),
+)