Fix FEC wrong payload type
Fix payload type number assignement for special payloads and flexfec: search if a number has already been assigned.
The test "Asymmetrical video call with flexfec starts with video" in test suite "video call" has been added to check that the repair packets of the FEC are correctly send to the FEC session during an asymetrical video start.
(cherry picked from commit de0855f6)
Edited by Flore Harlé