Make overlay video rendering opt-out
Make overlay video rendering opt-out

Closes #532.

multipath-rtp/openwebrtc,shoreflyer/openwebrtc,multipath-rtp/openwebrtc,shoreflyer/openwebrtc,EricssonResearch/openwebrtc,EricssonResearch/openwebrtc,EricssonResearch/openwebrtc,philn/openwebrtc,alessandrod/openwebrtc,stefhak/openwebrtc,stefhak/openwebrtc,alessandrod/openwebrtc,philn/openwebrtc,alessandrod/openwebrtc,alessandrod/openwebrtc,pererikb/openwebrtc,pererikb/openwebrtc,EricssonResearch/openwebrtc,stefhak/openwebrtc,alessandrod/openwebrtc,stefhak/openwebrtc,multipath-rtp/openwebrtc,shoreflyer/openwebrtc,pererikb/openwebrtc,philn/openwebrtc,pererikb/openwebrtc,stefhak/openwebrtc,multipath-rtp/openwebrtc,philn/openwebrtc,shoreflyer/openwebrtc,shoreflyer/openwebrtc,philn/openwebrtc,EricssonResearch/openwebrtc,philn/openwebrtc,EricssonResearch/openwebrtc,shoreflyer/openwebrtc,pererikb/openwebrtc,shoreflyer/openwebrtc,pererikb/openwebrtc,multipath-rtp/openwebrtc,multipath-rtp/openwebrtc,pererikb/openwebrtc,multipath-rtp/openwebrtc,EricssonResearch/openwebrtc,stefhak/openwebrtc,philn/openwebrtc,alessandrod/openwebrtc