vaapipostproc: Tweak the output video format setting to enable the autoplugging
vaapipostproc: Tweak the output video format setting to enable the autoplugging

This is a workaround until auto-plugging is fixed when
format=ENCODED + memory:VASurface caps feature are provided.

Use the downstream negotiated video format as the output video format
if the user didn't ask for the colorspace conversion explicitly.
Usecase: This will help to connect elements like videoscale, videorate etc
to vaapipostproc.

https://bugzilla.gnome.org/show_bug.cgi?id=739443

GStreamer/gstreamer-vaapi,ceyusa/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,GStreamer/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,ceyusa/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,CapOM/gstreamer-vaapi,CapOM/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,CapOM/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi