Clean-up: input thread: avoid bypassing demuxer straight to stream

In a number of cases, the input thread (src/input/input.c) messes with the byte stream stream in the back of the demuxer.

This is not only architecturally ugly. It prevents usage of dedicated threads within demuxers (the stream must be used on the input thread). At the time of filing, we have no such cases but it could change for instance to support time-based seeking for HTTP live streams.