This is now possible since OutputStream is a child of OutputFile and the
code allocating it can access MuxStream. Avoids the overhead and extra
complexity of allocating two objects instead of one.
Similar to what was previously done for OutputFile/Muxer.
Replace it with an array of streams in each OutputFile. This is a more
accurate reflection of the actual relationship between OutputStream and
OutputFile. This is easier to handle and will allow further
simplifications in future commits.
This is now possible since the code allocating OutputFile can see
sizeof(Muxer). Avoids the overhead and extra complexity of allocating
two objects instead of one.
Similar to what is done e.g. for AVStream/FFStream in lavf.
