In order to know the chroma format, besides profile, subsampling_x and
subsampling_y are needed (Spec 7.2.2 Color config semantics). These values are
in GstVp9Parser but not in GstVp9Framehdr.
Also, bit_depth is available in parser but not frame header. Evenmore, those
values are copied to picture structure later.
In case of VA-API, to configure the pipeline, it is require to know the chroma
format and depth.
It is possible to know chroma and depth through caps coming from vp9parser, but
it requires string parsing. It would be less error prone to get these values
through the parser structure at new_sequence() virtual method.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1700>
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