transport agent: optional OpenGL support
transport agent: optional OpenGL support

Add gldownload before the encoder to allow both OpenGL and non-OpenGL sources

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