va: vpp: Add raw buffer copy when needed.

va: vpp: Add raw buffer copy when needed.

Just like the decoder, the vapostproc also needs to copy the output
buffer to raw buffer if downstream elements only supports raw caps
and does not support the video meta.

The pipeline like:
  gst-launch-1.0 filesrc location=xxxx ! h264parse ! vah264dec ! \
  vapostproc ! capsfilter caps=video/x-raw,width=55,height=128 ! \
  filesink location=xxx
needs this logic to dump the data correctly.

fixes: #1523

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2026>