Add RTP/MJPEG decoder and encoder (#165)

* rename format.JPEG into format.MJPEG

* add examples/client-publish-format-mjpeg

* add rtp/mjpeg decoder and encoder

examples/client-read-format-mjpeg/main.go

@@ -0,0 +1,87 @@
+package main
+
+import (
+	"bytes"
+	"image/jpeg"
+	"log"
+
+	"github.com/aler9/gortsplib/v2"
+	"github.com/aler9/gortsplib/v2/pkg/format"
+	"github.com/aler9/gortsplib/v2/pkg/formatdecenc/rtpmjpeg"
+	"github.com/aler9/gortsplib/v2/pkg/url"
+	"github.com/pion/rtp"
+)
+
+// This example shows how to
+// 1. connect to a RTSP server
+// 2. check if there's a MJPEG media
+// 3. get JPEG images of that media
+// 4. decode JPEG images into raw images
+
+func main() {
+	c := gortsplib.Client{}
+
+	// parse URL
+	u, err := url.Parse("rtsp://localhost:8554/mystream")
+	if err != nil {
+		panic(err)
+	}
+
+	// connect to the server
+	err = c.Start(u.Scheme, u.Host)
+	if err != nil {
+		panic(err)
+	}
+	defer c.Close()
+
+	// find published medias
+	medias, baseURL, _, err := c.Describe(u)
+	if err != nil {
+		panic(err)
+	}
+
+	// find the MJPEG media and format
+	var forma *format.MJPEG
+	medi := medias.FindFormat(&forma)
+	if medi == nil {
+		panic("media not found")
+	}
+
+	// setup RTP/MJPEG->MJPEG decoder
+	rtpDec := forma.CreateDecoder()
+
+	// setup the chosen media only
+	_, err = c.Setup(medi, baseURL, 0, 0)
+	if err != nil {
+		panic(err)
+	}
+
+	// called when a RTP packet arrives
+	c.OnPacketRTP(medi, forma, func(pkt *rtp.Packet) {
+		// convert RTP packets into JPEG images
+		enc, pts, err := rtpDec.Decode(pkt)
+		if err != nil {
+			if err != rtpmjpeg.ErrNonStartingPacketAndNoPrevious && err != rtpmjpeg.ErrMorePacketsNeeded {
+				log.Printf("ERR: %v", err)
+			}
+			return
+		}
+
+		// convert JPEG images into raw images
+		image, err := jpeg.Decode(bytes.NewReader(enc))
+		if err != nil {
+			panic(err)
+		}
+
+		log.Printf("decoded image with size %v and pts %v", image.Bounds().Max, pts)
+	})
+
+	// start playing
+	_, err = c.Play(nil)
+	if err != nil {
+		panic(err)
+	}
+
+	// wait until a fatal error
+	panic(c.Wait())
+}