Improve algorithm to add a new stream to the SDP to take into account shift by more than one position when appending.
If a stream must be placed at a precise location then try to find the best place to move the stream currently occupying this slot:
- append if there is no inactive stream and it was not found in the previous media description
- take slot of an inactive stream if it was not found in the previous media description - this slot may be reused later by a following stream or be one of a participant that left the conference
- take same position as in the previous media description if it was already present there