vaapiencode: flush pending frames before set format
vaapiencode: flush pending frames before set format

Flush pending frames, if any, in the internal encorder, before setting
the new negotiated format.

https://bugzilla.gnome.org/show_bug.cgi?id=786173

ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi,GStreamer/gstreamer-vaapi,ceyusa/gstreamer-vaapi