Allow configuration of ffmpeg args (#3216)

* Allow configuration of ffmpeg args
* Add UI settings for ffmpeg config
* Add changelog entry
* Add documentation in manual

pkg/ffmpeg/stream.go

@@ -144,11 +144,17 @@ type TranscodeStreamOptions struct {
 	// in some videos where the audio codec is not supported by ffmpeg
 	// ffmpeg fails if you try to transcode the audio
 	VideoOnly bool
+
+	// arguments added before the input argument
+	ExtraInputArgs []string
+	// arguments added before the output argument
+	ExtraOutputArgs []string
 }
 
 func (o TranscodeStreamOptions) getStreamArgs() Args {
 	var args Args
 	args = append(args, "-hide_banner")
+	args = append(args, o.ExtraInputArgs...)
 	args = args.LogLevel(LogLevelError)
 
 	if o.StartTime != 0 {
@@ -184,6 +190,8 @@ func (o TranscodeStreamOptions) getStreamArgs() Args {
 		"-ac", "2",
 	)
 
+	args = append(args, o.ExtraOutputArgs...)
+
 	args = args.Format(o.Codec.format)
 	args = args.Output("pipe:")