#29579: video mixing implementation and conference fixes.
- mixer rendering implemention => frame based (was per sources batch based) - add backward signaling to Observer/Obsevable classes => This help mixer to index sources for layout them. - mutex'ed frame publish (VideoGenerator). - sinks creation are now done at right places. => one per mixer (new), one per camera, one per stream reception. - VideoRTPSession is fully responsible to handle video pipeline, between RX/TS streams. => exhibit enterConference/exitConference to be aknowledged by upper layers. - VideoSendThread is not longer a «thread», renamed as VideoSender. - videoMixer_ is now a shared ptr in Conference objects. => getVideoMixer() return a rew shared_ptr also. - Conference is now responsible to trig video conference pipeline - std::this_thread::sleep_for() is not usable before GCC 4.1