vaapidecodebin: capsfilter to optimize negotiation
vaapidecodebin: capsfilter to optimize negotiation

Add a capsfilter forcing the caps
"video/x-raw(memory:VASurface), format=(string)NV12" between the
queue and the vaapipostproc so no renegotiation is required.

https://bugzilla.gnome.org/show_bug.cgi?id=776175

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