Add VNC as a video source (#372)

Add VNC as a video source

examples/vnc/README.md

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+## Instructions
+
+### Install required codecs
+
+In this example, we'll be using x264 and opus as our video and audio codecs. Therefore, we need to make sure that these codecs are installed within our system. 
+
+Installation steps:
+
+* [x264](https://github.com/pion/mediadevices#x264)
+
+### Download vnc example
+
+```
+git clone https://github.com/pion/mediadevices.git
+```
+
+#### Compile vnc example
+
+```
+cd mediadevices/examples/vnc && go build
+```
+
+### Open example page
+
+[jsfiddle.net](https://jsfiddle.net/gh/get/library/pure/pion/mediadevices/tree/master/examples/internal/jsfiddle/audio-and-video) you should see two text-areas and a 'Start Session' button
+
+### Run the webrtc example with your browsers SessionDescription as stdin
+
+In the jsfiddle the top textarea is your browser, copy that, and store the session description in an environment variable, `export SDP=<put_the_sdp_here>`
+
+Run `echo $SDP | ./vnc`
+
+In Windows
+
+```powershell
+type sdp.txt| .\vnc.exe
+```
+### Input webrtc's SessionDescription into your browser
+
+Copy the text that `./webrtc` just emitted and copy into second text area
+
+### Hit 'Start Session' in jsfiddle, enjoy your video!
+
+A video should start playing in your browser above the input boxes, and will continue playing until you close the application.
+
+Congrats, you have used pion-MediaDevices! Now start building something cool

examples/vnc/main.go

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+package main
+
+import (
+	"fmt"
+	"github.com/pion/mediadevices/pkg/driver"
+	"github.com/pion/mediadevices/pkg/driver/vncdriver"
+
+	"github.com/pion/mediadevices"
+	"github.com/pion/mediadevices/examples/internal/signal"
+	"github.com/pion/webrtc/v3"
+
+	// If you don't like x264, you can also use vpx by importing as below
+	// "github.com/pion/mediadevices/pkg/codec/vpx" // This is required to use VP8/VP9 video encoder
+	// or you can also use openh264 for alternative h264 implementation
+	// "github.com/pion/mediadevices/pkg/codec/openh264"
+	// or if you use a raspberry pi like, you can use mmal for using its hardware encoder
+	// "github.com/pion/mediadevices/pkg/codec/mmal"
+	"github.com/pion/mediadevices/pkg/codec/x264" // This is required to use h264 video encoder
+
+	// Note: If you don't have a camera or microphone or your adapters are not supported,
+	//       you can always swap your adapters with our dummy adapters below.
+	// _ "github.com/pion/mediadevices/pkg/driver/videotest"
+	// _ "github.com/pion/mediadevices/pkg/driver/audiotest"
+)
+
+func main() {
+	config := webrtc.Configuration{
+		ICEServers: []webrtc.ICEServer{
+			{
+				URLs: []string{"stun:stun.l.google.com:19302"},
+			},
+		},
+	}
+	driver.GetManager().Register(
+		vncdriver.NewVnc("127.0.0.1:5900"),
+		driver.Info{Label: "VNC", DeviceType: driver.Camera, Priority: driver.PriorityLow},
+	)
+	// Wait for the offer to be pasted
+	offer := webrtc.SessionDescription{}
+	signal.Decode(signal.MustReadStdin(), &offer)
+
+	// Create a new RTCPeerConnection
+	x264Params, err := x264.NewParams()
+	if err != nil {
+		panic(err)
+	}
+	x264Params.BitRate = 500_000 // 500kbps
+
+	if err != nil {
+		panic(err)
+	}
+	codecSelector := mediadevices.NewCodecSelector(
+		mediadevices.WithVideoEncoders(&x264Params),
+	)
+
+	mediaEngine := webrtc.MediaEngine{}
+	codecSelector.Populate(&mediaEngine)
+	api := webrtc.NewAPI(webrtc.WithMediaEngine(&mediaEngine))
+	peerConnection, err := api.NewPeerConnection(config)
+	if err != nil {
+		panic(err)
+	}
+
+	// Set the handler for ICE connection state
+	// This will notify you when the peer has connected/disconnected
+	peerConnection.OnICEConnectionStateChange(func(connectionState webrtc.ICEConnectionState) {
+		fmt.Printf("Connection State has changed %s \n", connectionState.String())
+	})
+
+	s, err := mediadevices.GetUserMedia(mediadevices.MediaStreamConstraints{
+		Video: func(c *mediadevices.MediaTrackConstraints) {
+
+		},
+		Codec: codecSelector,
+	})
+	if err != nil {
+		panic(err)
+	}
+
+	for _, track := range s.GetTracks() {
+		track.OnEnded(func(err error) {
+			fmt.Printf("Track (ID: %s) ended with error: %v\n",
+				track.ID(), err)
+		})
+
+		_, err = peerConnection.AddTransceiverFromTrack(track,
+			webrtc.RtpTransceiverInit{
+				Direction: webrtc.RTPTransceiverDirectionSendonly,
+			},
+		)
+		if err != nil {
+			panic(err)
+		}
+	}
+
+	// Set the remote SessionDescription
+	err = peerConnection.SetRemoteDescription(offer)
+	if err != nil {
+		panic(err)
+	}
+
+	// Create an answer
+	answer, err := peerConnection.CreateAnswer(nil)
+	if err != nil {
+		panic(err)
+	}
+
+	// Create channel that is blocked until ICE Gathering is complete
+	gatherComplete := webrtc.GatheringCompletePromise(peerConnection)
+
+	// Sets the LocalDescription, and starts our UDP listeners
+	err = peerConnection.SetLocalDescription(answer)
+	if err != nil {
+		panic(err)
+	}
+
+	// Block until ICE Gathering is complete, disabling trickle ICE
+	// we do this because we only can exchange one signaling message
+	// in a production application you should exchange ICE Candidates via OnICECandidate
+	<-gatherComplete
+
+	// Output the answer in base64 so we can paste it in browser
+	fmt.Println(signal.Encode(*peerConnection.LocalDescription()))
+
+	// Block forever
+	select {}
+}