Modifying read-only buffer error

I use the sample in your repository (https://github.com/OnlyInAmerica/FFmpegTest) to test muxing by ffmpeg, but it's always get exception.
The reason is you modify output buffer of the video encoder in HLSRecorder.drainEncoder(), which is a read-only buffer.

Can you tell me what I have to change to make it work?

Thank you very much!
