exif: Allow more spacing characters in strings

The root cause of issue #8079 was a non-breaking space (U+0160).
`unicode.IsPrint` only allows the ASCII space (U+0020).  Be more lenient
by using `unicode.IsGraphic` instead.

Fixes #8079

resources/images/exif/exif.go

@@ -227,7 +227,7 @@ func (e *exifWalker) Walk(f _exif.FieldName, tag *tiff.Tag) error {
 func nullString(in []byte) string {
 	var rv bytes.Buffer
 	for _, b := range in {
-		if unicode.IsPrint(rune(b)) {
+		if unicode.IsGraphic(rune(b)) {
 			rv.WriteByte(b)
 		}
 	}

resources/images/exif/exif_test.go

@@ -75,6 +75,20 @@ func TestExifPNG(t *testing.T) {
 	c.Assert(err, qt.Not(qt.IsNil))
 }
 
+func TestIssue8079(t *testing.T) {
+	c := qt.New(t)
+
+	f, err := os.Open(filepath.FromSlash("../../testdata/iss8079.jpg"))
+	c.Assert(err, qt.IsNil)
+	defer f.Close()
+
+	d, err := NewDecoder()
+	c.Assert(err, qt.IsNil)
+	x, err := d.Decode(f)
+	c.Assert(err, qt.IsNil)
+	c.Assert(x.Tags["ImageDescription"], qt.Equals, "Città del Vaticano #nanoblock #vatican #vaticancity")
+}
+
 func BenchmarkDecodeExif(b *testing.B) {
 	c := qt.New(b)
 	f, err := os.Open(filepath.FromSlash("../../testdata/sunset.jpg"))