conference: handle participants without video

Conferences informations were missing two cases:
1. Somebody in the conference, but without any video session. This participant
is only present in the conference object and need to be added in the informations
sent.
2. A participant with a video output, but no input (video muted). In this case,
the video is rendered by the video mixer, but with a black frame. The coordinates
should be added into the infos.

To do that, the position calculation is done outside the render_frame and
ParticipantInfos now has a videoMuted and audioMuted (not used for now) field

Change-Id: I0b979f99c9db032dccbbc8a2cd1a14125ef72071