Add DASH streams for VP9 transcoding (#3275)

vendor/github.com/zencoder/go-dash/v3/mpd/pssh.go

@@ -0,0 +1,41 @@
+package mpd
+
+import (
+	"bytes"
+	"encoding/binary"
+	"fmt"
+)
+
+func MakePSSHBox(systemID, payload []byte) ([]byte, error) {
+	if len(systemID) != 16 {
+		return nil, fmt.Errorf("SystemID must be 16 bytes, was: %d", len(systemID))
+	}
+
+	psshBuf := &bytes.Buffer{}
+	size := uint32(12 + 16 + 4 + len(payload)) // 3 uint32s, systemID, "pssh" string and payload
+	if err := binary.Write(psshBuf, binary.BigEndian, size); err != nil {
+		return nil, err
+	}
+
+	if err := binary.Write(psshBuf, binary.BigEndian, []byte("pssh")); err != nil {
+		return nil, err
+	}
+
+	if err := binary.Write(psshBuf, binary.BigEndian, uint32(0)); err != nil {
+		return nil, err
+	}
+
+	if _, err := psshBuf.Write(systemID); err != nil {
+		return nil, err
+	}
+
+	if err := binary.Write(psshBuf, binary.BigEndian, uint32(len(payload))); err != nil {
+		return nil, err
+	}
+
+	if _, err := psshBuf.Write(payload); err != nil {
+		return nil, err
+	}
+
+	return psshBuf.Bytes(), nil
+}