Add thin compatibility layer for the deprecated GstVideoContext API.
For GStreamer API >= 1.2, this involves the following two functions:
- gst_vaapi_video_context_prepare(): queries if a context is already
set in the pipeline ;
- gst_vaapi_video_context_propagate(): propagates the newly-created
context to the rest of the pipeline.
https://bugzilla.gnome.org/show_bug.cgi?id=703235
Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>
