panfrost: Add optional opaque packs to GenXML

panfrost: Add optional opaque packs to GenXML

These special OPAQUE packs use packed structs in the struct template,
instead of struct templates. The use case is packing nested structs
out-of-band, to fit into the CSO model.

A more conventional GenXML solution would be an overlapping uint, but
this breaks our assumptions about struct packing which are otherwise
correct, so this seemed less intrusive than risk disrupting the main
pack routines.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6440>