stream/mts & stream/rtp: fixed timing calculations

Currently time calculations in the mts encoder are based on the premise that each mpegts packet is a 'frame', and in the rtp encoder that
each encoding of multiple packets into one rtp packet is a 'frame', these are both false. Mpegts encoding and rtp encoding should do
timestamp calculations using a reasonable approximation of time at encoding/send.

stream/mts/encoder.go

@@ -122,6 +122,7 @@ type Encoder struct {
 	dst io.Writer
 
 	clock         time.Duration
+	lastTime      time.Time
 	frameInterval time.Duration
 	ptsOffset     time.Duration
 	tsSpace       [PacketSize]byte
@@ -247,7 +248,9 @@ func (e *Encoder) writePSI() error {
 
 // tick advances the clock one frame interval.
 func (e *Encoder) tick() {
-	e.clock += e.frameInterval
+	now := time.Now()
+	e.clock += now.Sub(e.lastTime)
+	e.lastTime = now
 }
 
 // pts retuns the current presentation timestamp.

stream/rtp/encoder.go

@@ -51,6 +51,7 @@ type Encoder struct {
 	seqNo         uint16
 	clock         time.Duration
 	frameInterval time.Duration
+	lastTime      time.Time
 	fps           int
 	buffer        []byte
 	pktSpace      [defPktSize]byte
@@ -120,7 +121,9 @@ func (e *Encoder) Encode(payload []byte) error {
 
 // tick advances the clock one frame interval.
 func (e *Encoder) tick() {
-	e.clock += e.frameInterval
+	now := time.Now()
+	e.clock += now.Sub(e.lastTime)
+	e.lastTime = now
 }
 
 // nxtTimestamp gets the next timestamp