Consistently use SPS for detecting H.264 keyframes.

This was already the case for FU NALUs, but not for simple NALUs.

codecs/codecs.go

@@ -125,7 +125,7 @@ func Keyframe(codec string, packet *rtp.Packet) (bool, bool) {
 			return false, false
 		} else if nalu <= 23 {
 			// simple NALU
-			return nalu == 5, true
+			return nalu == 7, true
 		} else if nalu == 24 || nalu == 25 || nalu == 26 || nalu == 27 {
 			// STAP-A, STAP-B, MTAP16 or MTAP24
 			i := 1