owr_video_renderer: Make glvideoflip support optional
owr_video_renderer: Make glvideoflip support optional

Due to the dependency on Graphene (not widely packaged in distros yet)
it isn't a good idea to have a hard dependency on the glvideoflip
element.

Closes #629.

pererikb/openwebrtc,shoreflyer/openwebrtc,shoreflyer/openwebrtc,EricssonResearch/openwebrtc,shoreflyer/openwebrtc,pererikb/openwebrtc,pererikb/openwebrtc,EricssonResearch/openwebrtc,shoreflyer/openwebrtc,shoreflyer/openwebrtc,pererikb/openwebrtc,pererikb/openwebrtc,shoreflyer/openwebrtc,EricssonResearch/openwebrtc,EricssonResearch/openwebrtc,shoreflyer/openwebrtc,pererikb/openwebrtc,EricssonResearch/openwebrtc,pererikb/openwebrtc,EricssonResearch/openwebrtc,EricssonResearch/openwebrtc