This commit adds 3 new properties to split output file in the output
`ffmpeg_muxer`.
- `max_time_sec` specifies the limit in seconds.
- `max_size_mb` specifies the limit in megabytes.
- `allow_overwrite` specifies to test an existing file.
If both `max_time_sec` and `max_size_mb` are not positive, the split
file feature won't be enabled.
Another output ffmpeg_mpegts_muxer shares the code but is not affected
since the output is used only for streaming.
ce92f441b5