ges-pipeline: Should be able to handle GstContext like playbin
playsink checks whether videosink is usable or not by setting GST_STATE_READY. And playbin{2,3} has special handling for GstContext
around it.
ges-pipeline needs to implement it as well.
See also gst-plugins-base!531 (merged) and gst-plugins-base!384 (merged)