UnicodeDecodeError: 'utf-8' codec can't decode byte 0xff in position ...: invalid start byte

When a video has a non utf-8 metadate the encoding breaks. This is the output of ffmpeg:

```
    com.apple.quicktime.artwork: ����
```

And here is where the encoding breaks:

```
video_encoding/backends/ffmpeg.py", line 97, in encode
line = reader.readline()
```

I think there should be no problem to ignore invalid characters of ffmpeg is happy to process the file?