Decode streams once and only when needed (#1218)

* split data into specialized structs

* move MPEG4-audio decoding into streamTrack

* restore video/audio synchronization in HLS muxer and RTMP server

* log decode errors

* move H264 decoding and re-encoding here from gortsplib

* add tests

* update gortsplib

internal/core/reader.go

@@ -3,6 +3,6 @@ package core
 // reader is an entity that can read a stream.
 type reader interface {
 	close()
-	onReaderData(*data)
+	onReaderData(data)
 	apiReaderDescribe() interface{}
 }