Redundant stdin sink

[dexterlb]

The stdin sink superficially looks as if it subsumes the ffmpeg_stdin sink - maybe we can delete one of them?
The stdin/stdout sources and sinks may be refactored and streamlined to be more generic somehow…

[MartijnBraam]

The main difference is the texture types and a bit of ffmpeg handling, I think they can probably be merged together into one flexible pipe output sink