remove h264 decoding and re-encoding (#144)

* stop re-encoding H264

* stop decoding H264

* improve tests

ptsequalsdts.go

@@ -0,0 +1,71 @@
+package gortsplib
+
+import (
+	"github.com/pion/rtp"
+
+	"github.com/aler9/gortsplib/pkg/h264"
+)
+
+// find IDR NALUs without decoding RTP
+func rtpH264ContainsIDR(pkt *rtp.Packet) bool {
+	if len(pkt.Payload) == 0 {
+		return false
+	}
+
+	typ := h264.NALUType(pkt.Payload[0] & 0x1F)
+
+	switch typ {
+	case h264.NALUTypeIDR:
+		return true
+
+	case 24: // STAP-A
+		payload := pkt.Payload[1:]
+
+		for len(payload) > 0 {
+			if len(payload) < 2 {
+				return false
+			}
+
+			size := uint16(payload[0])<<8 | uint16(payload[1])
+			payload = payload[2:]
+
+			if size == 0 || int(size) > len(payload) {
+				return false
+			}
+
+			nalu := payload[:size]
+			payload = payload[size:]
+
+			typ = h264.NALUType(nalu[0] & 0x1F)
+			if typ == h264.NALUTypeIDR {
+				return true
+			}
+		}
+
+		return false
+
+	case 28: // FU-A
+		if len(pkt.Payload) < 2 {
+			return false
+		}
+
+		start := pkt.Payload[1] >> 7
+		if start != 1 {
+			return false
+		}
+
+		typ := h264.NALUType(pkt.Payload[1] & 0x1F)
+		return (typ == h264.NALUTypeIDR)
+
+	default:
+		return false
+	}
+}
+
+func ptsEqualsDTS(track Track, pkt *rtp.Packet) bool {
+	if _, ok := track.(*TrackH264); ok {
+		return rtpH264ContainsIDR(pkt)
+	}
+
+	return true
+}