[transcoder] Copy the least prioritized stream to a tmp-stream for later usage on timeouts.

[Fusl]

When the source function waits for a timeout to occur (say 10 seconds), there will be a gap on the output stream of 10 seconds then. To omit this, we want to copy the least prioritized running, readable and stable stream to a buffer for each mount and on each byte which has been read truncate the buffer to 0. When the timeout occurs, the buffer will be filled with X seconds (X is the time how long to wait until the timeout occurs) and can then be copied very fast into the mount input. But watch out that this must skip the overflow unsyncdiscard check! (Maybe temporarily disable the mounts[mountpoint].unsyncdiscard parameter and after filling it with the buffer data, enable again?)