Add basic video support to WHIP. (#3602)

Tested with eyevinn client.

There are a few issues to figure out still
1. Simulcast - how?
2. For simulcast, how to know width, height so that adaptive stream can
   work.
3. The layer added is dummy. It works, but connection quality scoring
   would be incorrect (will always say excellent) without bitrate.

Will need some dynamic update of `TrackInfo` based on actual stream for
all of this to fit well into our system, but the simple video support
works for now.

pkg/rtc/participant.go

@@ -858,8 +858,7 @@ func (p *ParticipantImpl) synthesizeAddTrackRequests(offer webrtc.SessionDescrip
 	}
 
 	for _, m := range parsed.MediaDescriptions {
-		if !strings.EqualFold(m.MediaName.Media, "audio") {
-			// ONE-SHOT-SIGNALLING-MODE-TODO: support video
+		if !strings.EqualFold(m.MediaName.Media, "audio") && !strings.EqualFold(m.MediaName.Media, "video") {
 			continue
 		}
 
@@ -886,15 +885,34 @@ func (p *ParticipantImpl) synthesizeAddTrackRequests(offer webrtc.SessionDescrip
 			trackID = guid.New(utils.TrackPrefix)
 		}
 
+		var (
+			name        string
+			trackSource livekit.TrackSource
+			trackType   livekit.TrackType
+		)
+		if strings.EqualFold(m.MediaName.Media, "audio") {
+			name = "synthesized-microphone"
+			trackSource = livekit.TrackSource_MICROPHONE
+			trackType = livekit.TrackType_AUDIO
+		} else {
+			name = "synthesized-camera"
+			trackSource = livekit.TrackSource_CAMERA
+			trackType = livekit.TrackType_VIDEO
+		}
 		req := &livekit.AddTrackRequest{
 			Cid:        trackID,
-			Name:       "synthesized-microphone",
-			Source:     livekit.TrackSource_MICROPHONE,
-			Type:       livekit.TrackType_AUDIO,
+			Name:       name,
+			Source:     trackSource,
+			Type:       trackType,
 			DisableDtx: true,
 			Stereo:     false,
 			Stream:     "camera",
 		}
+		// ONE-SHOT-SIGNALLING-MODE-TODO: support video simulcast
+		if strings.EqualFold(m.MediaName.Media, "video") {
+			// dummy layer to ensure at least one layer is available
+			req.Layers = []*livekit.VideoLayer{{}}
+		}
 		p.AddTrack(req)
 	}
 	return nil