It's not just position that's special here, we also need space for
other special vertex-shader outputs.
This fixes a crash with Zink on Lavapipe, because Zink always emits
psize to avoid some shader-keys. While this might not be ideal, it's not
illegal. So we need to handle it.
But it seems we're missing more, and this list is taken from
compute_vertex_info() in lp_state_derived.c.
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10730>
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