vaapipostproc: add fixate_caps() vmethod
vaapipostproc: add fixate_caps() vmethod

Instead of fixating the srcpad caps in transform_caps() vmethod, this patch
implements the fixate_caps() vmethod and moves code around.

https://bugzilla.gnome.org/show_bug.cgi?id=758548

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