add CreateDecoder() method to all tracks that can be decoded

examples/client-read-codec-h264-convert-to-jpeg/main.go

@@ -9,7 +9,6 @@ import (
 	"time"
 
 	"github.com/aler9/gortsplib"
-	"github.com/aler9/gortsplib/pkg/rtph264"
 	"github.com/aler9/gortsplib/pkg/url"
 )
 
@@ -75,10 +74,7 @@ func main() {
 	}
 
 	// setup RTP/H264->H264 decoder
-	rtpDec := &rtph264.Decoder{
-		PacketizationMode: track.PacketizationMode,
-	}
-	rtpDec.Init()
+	rtpDec := track.CreateDecoder()
 
 	// setup H264->raw frames decoder
 	h264RawDec, err := newH264Decoder()