vaapipostproc: set GST_VAAPI_POSTPROC_FLAG_SIZE according to src caps
vaapipostproc: set GST_VAAPI_POSTPROC_FLAG_SIZE according to src caps

A value of width/height property should be set to out caps,
if negotiation had been going properly.
So we can use srcpad_info when making decision of scaling.

https://bugzilla.gnome.org/show_bug.cgi?id=778010

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