This new struct describes the input and output GstBuffers to
post-process, including VA flags. It also contains the VASurfaceID and
VARectangle, but those are private, completed inside GstVaFilter.
It is used for pass arguments to gst_va_filter_convert_surface() function.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2058>