Add new gst_vaapi_video_buffer_new() helper function that allocates a video
buffer from a GstVaapiVideoMeta. Also remove obsolete and useless function
gst_vaapi_video_buffer_get_meta().
Move GstVaapiVideoMeta from core libgstvaapi decoding library to the
actual plugin elements. That's only useful there. Also inline reference
counting code from GstVaapiMiniObject.