[Libva] Add passthrough mode to upload/download elements

[Simon Farnsworth]

I'm currently integrating VA-API support into an existing GStreamer
application, using playbin2. I want to use vaapisink for output, uploading
software decoded video to VA-API; while playbin2 lets me set a bin as the
video-sink property, it doesn't let me choose elements depending on what the
input is.

The easy fix was to make vaapiupload capable of passthrough, so that I can
unconditionally set my playbin2 video-sink as "vaapiupload ! vaapipostproc !
vaapisink" and let vaapiupload enter passthrough mode when playbin2 chooses
vaapidecode as the decoder.

I also made the corresponding changes to vaapidownload; I don't personally
need them, as I only use vaapidownload when debugging, but the changes
aren't complex.