scaling in vaapipostproc
I think the current vaapipostproc would only do the cropping not the scaling. And scaling is done in vaapisink. But I don't think the vaapisink is available in many situation(for example, iHD doesn't offer wayland support).
In the most of case, the downstream is just another gstreamer videosink, although we can expect the videosink would use the hardware scaling to down or up scaling. But in Intel Broxton, down scaling for UHD video is somehow impossible for DRM. So it would be better to use the VPP to do that here.