Fix parsing of some modern ICNS files (#2)

* Fix parsing to skip complete sections

TOC is variable length
Icons that are not recognised should be skipped as well
Fixes rendering of Apple Mail and other modern icons

* Don't even try to parse JPEG2000 images

* Tidy up code for skipping JPEG2000

* Clean up naming

reader.go

@@ -11,6 +11,8 @@ import (
 	"github.com/pkg/errors"
 )
 
+var jpeg2000header = []byte{0x00, 0x00, 0x00, 0x0c, 0x6a, 0x50, 0x20, 0x20}
+
 // Decode finds the largest icon listed in the icns file and returns it,
 // ignoring all other sizes. The format returned will be whatever the icon data
 // is, typically jpeg or png.
@@ -31,17 +33,28 @@ func Decode(r io.Reader) (image.Image, error) {
 		read += 4
 		switch string(next) {
 		case "TOC ":
-			read += 4
+			tocSize := binary.BigEndian.Uint32(data[read : read+4])
+			read += tocSize-4 // size includes header and size fields
 			continue
 		case "icnV":
 			read += 4
 			continue
 		}
+
+		dataSize := binary.BigEndian.Uint32(data[read : read+4])
+		read += 4
+		if dataSize == 0 {
+			continue // no content, we're not interested
+		}
+
+		iconData := data[read : read+dataSize-8]
+		read += dataSize-8 // size includes header and size fields
+
 		if isOsType(string(next)) {
-			iconSize := binary.BigEndian.Uint32(data[read : read+4])
-			read += 4
-			iconData := data[read : read+iconSize]
-			read += iconSize
+			if bytes.Equal(iconData[:8], jpeg2000header) {
+				continue // skipping JPEG2000
+			}
+
 			icons = append(icons, iconReader{
 				OsType: osTypeFromID(string(next)),
 				r:      bytes.NewBuffer(iconData),