Add NewEncodedReeader API

Changes:
  * Add NewEncodedReeader method to Track interface
  * Add video archival example

examples/archive/README.md

@@ -0,0 +1,36 @@
+## Instructions
+
+### Install required codecs
+
+In this example, we'll be using x264 as our video codec. Therefore, we need to make sure that these codecs are installed within our system. 
+
+Installation steps:
+
+* [x264](https://github.com/pion/mediadevices#x264)
+
+### Download archive examplee
+
+```
+git clone https://github.com/pion/mediadevices.git
+```
+
+### Run archive example
+
+Run `cd mediadevices/examples/archive && go build && ./archive recorded.h264`
+
+### Playback recorded video
+
+Install GStreamer and run:
+```
+gst-launch-1.0 playbin uri=file://${PWD}/recorded.h264
+```
+
+Or run VLC media plyer:
+```
+vlc recorded.h264
+```
+
+A video should start playing in your GStreamer or VLC window.
+
+Congrats, you have used pion-MediaDevices! Now start building something cool
+

examples/archive/main.go

@@ -0,0 +1,82 @@
+package main
+
+import (
+	"fmt"
+	"image"
+	"io"
+	"os"
+	"os/signal"
+	"syscall"
+
+	"github.com/pion/mediadevices"
+	"github.com/pion/mediadevices/pkg/codec/x264"      // This is required to use H264 video encoder
+	_ "github.com/pion/mediadevices/pkg/driver/camera" // This is required to register camera adapter
+	"github.com/pion/mediadevices/pkg/frame"
+	"github.com/pion/mediadevices/pkg/io/video"
+	"github.com/pion/mediadevices/pkg/prop"
+)
+
+func must(err error) {
+	if err != nil {
+		panic(err)
+	}
+}
+
+func main() {
+	if len(os.Args) != 2 {
+		fmt.Printf("usage: %s <path/to/file.h264>\n", os.Args[0])
+		return
+	}
+	dest := os.Args[1]
+
+	sigs := make(chan os.Signal, 1)
+	signal.Notify(sigs, syscall.SIGINT)
+
+	x264Params, err := x264.NewParams()
+	must(err)
+	x264Params.Preset = x264.PresetMedium
+	x264Params.BitRate = 1_000_000 // 1mbps
+
+	codecSelector := mediadevices.NewCodecSelector(
+		mediadevices.WithVideoEncoders(&x264Params),
+	)
+
+	mediaStream, err := mediadevices.GetUserMedia(mediadevices.MediaStreamConstraints{
+		Video: func(c *mediadevices.MediaTrackConstraints) {
+			c.FrameFormat = prop.FrameFormat(frame.FormatYUY2)
+			c.Width = prop.Int(640)
+			c.Height = prop.Int(480)
+		},
+		Codec: codecSelector,
+	})
+	must(err)
+
+	videoTrack := mediaStream.GetVideoTracks()[0].(*mediadevices.VideoTrack)
+	defer videoTrack.Close()
+
+	videoTrack.Transform(video.TransformFunc(func(r video.Reader) video.Reader {
+		return video.ReaderFunc(func() (img image.Image, release func(), err error) {
+			// we send io.EOF signal to the encoder reader to stop reading. Therefore, io.Copy
+			// will finish its execution and the program will finish
+			select {
+			case <-sigs:
+				return nil, func() {}, io.EOF
+			default:
+			}
+
+			return r.Read()
+		})
+	}))
+
+	reader, err := videoTrack.NewEncodedReader(x264Params.RTPCodec().Name)
+	must(err)
+	defer reader.Close()
+
+	out, err := os.Create(dest)
+	must(err)
+
+	fmt.Println("Recording... Press Ctrl+c to stop")
+	_, err = io.Copy(out, reader)
+	must(err)
+	fmt.Println("Your video has been recorded to", dest)
+}