produce same absolute time in RTSP and HLS (#1249)

* add a NTP timestamp to each data unit
* use that NTP timestamp in all protocols

internal/core/rpicamera_source.go

@@ -62,6 +62,7 @@ func (s *rpiCameraSource) run(ctx context.Context) error {
 			trackID: 0,
 			pts:     dts,
 			nalus:   nalus,
+			ntp:     time.Now(),
 		})
 		if err != nil {
 			s.Log(logger.Warn, "%v", err)