RTSP_STREAM_AUDIO_ENABLED=true sets audio disabled

[culyzhao]

I found something wrong with https://github.com/riltech/streamer/blob/master/process.go

if p.audio { processCommands` = append(processCommands, "-an") }

-an means disable audio, so by default, the hls video has no audio, if sets RTSP_STREAM_AUDIO_ENABLED=false, -an will not be added to ffmpeg options, the hls audio plays normally.